Coated steel sheet

ABSTRACT

A coated steel sheet including a steel sheet and a coating layer provided on at least part of the surface of the steel sheet, in which the coating layer has a predetermined chemical composition in terms of % by mass; in which the coating layer has a laminar Mg 2 Sn phase-containing structure in an area fraction of from 5 to 65%, and a structure containing a solid solution of Zn and Al; and the laminar Mg 2 Sn phase-containing structure is a structure constituted with a Zn phase and a laminar Mg 2 Sn phase having a thickness of less than 1 μm, and in which the laminar Mg 2 Sn phase exists dividing the Zn phase into plural regions.

TECHNICAL FIELD

The present invention relates to a coated steel sheet.

BACKGROUND ART

In recent years, a coated steel sheet is used as an automobilestructural member from the viewpoint of rust prevention, and an alloyedhot-dip zinc-coated steel sheet is mainly applied in the Japanesemarket. The alloyed hot-dip zinc-coated steel sheet is a coated steelsheet for which firstly hot-dip zinc-coating is performed on a steelsheet and then an alloying heat treatment is performed thereon such thatthe weldability and corrosion resistance after painting are improved bymeans of diffusion of Fe from the steel sheet (substrate steel sheet) inthe coating layer. For example, a coated steel sheet described in PatentLiterature 1 is representatively used as a coated steel sheet forautomobiles in Japan.

Usually, a coated steel sheet for automobile is used in a state formedinto a complex shape from a sheet form, therefore it is in many casessubjected to press forming. In the case of an alloyed hot-dipzinc-coated steel sheet, the coating layer becomes hard due to diffusionof Fe from the substrate steel sheet. Therefore, the coating layer iseasily detached, and there is a unique problem such as powdering orflaking that is not experienced in a hot-dip zinc-coated steel sheethaving a soft coating layer.

Further, in the case of a coated steel sheet provided with a hardcoating layer, the coating layer is prone to be damaged by an externalpressure, and once a crack is generated, the crack propagates to aninterface between the coating layer and the steel substrate (steelsheet). And it is regarded as a problem that the coating layer is proneto be detached from the interface with the steel substrate (steel sheet)as a starting point to cause falling.

For example, when an alloyed hot-dip zinc-coated steel sheet is used foran automobile outer panel, the steel substrate (steel sheet) tends to beexposed due to simultaneous detachment of a paint and a coating layer byan impingement of a stone (chipping) kicked up by a traveling vehicle.Therefore, its corrosion advances faster than a coated steel sheetprovided with a soft coating layer which is not alloyed. For thisreason, in a case of use for an extended time period, erosion due tocorrosion of the steel substrate starts, so especially when it is usedas an underbody member, it is feared that the collision safetyperformance is deteriorated.

Furthermore, since the alloyed hot-dip zinc-coated steel sheet containsFe in the coating layer, if such chipping occurs, reddish-brown rust isreadily generated due to corrosion of the coating layer, which causes aproblem also on the appearance of an automobile.

As a solution to these problems, it is effective to apply a coated steelsheet with a coating layer having a favorable toughness and notcontaining Fe. For example, as an automobile coated steel sheet with acoating layer not containing Fe, a hot-dip zinc-coating steel sheet ismainly used in North America, Europe, etc. In this regard, a hot-dipzinc-coating steel sheet which has not undergone an alloying treatmentis resistant to chipping. In addition, since Fe is not contained in thecoating layer in contrast to an alloyed hot-dip zinc-coated steel sheet,red rust in the initial stage of corrosion is also less likely toappear. However, when it is painted, the coating layer is easilycorroded under the paint film to raise the paint film (blistering).Therefore, a hot-dip zinc-coating steel sheet is by no means suitablefor an automobile coated steel sheet, because the steel substrate alsostarts to be eroded when it is used for a long time period.

As a method of enhancing the corrosion resistance of a hot-dip Zncoating, there is, for example, a method of adding Al in the Zn coatinglayer, and in the building materials field, as disclosed in PatentLiterature 2, as a high corrosion resistance coated steel sheet, ahot-dip Al—Zn coated steel sheet is widely put to practical use. Thecoating layer of such a hot-dip Al—Zn coated steel sheet is constitutedwith a dendrite-like α-Al phase (dendritic structure) crystallized firstfrom the molten state, and a structure consisting of a Zn phase and anAl phase formed in the interstices of the dendritic structures(interdendritic structure). The dendritic structure is passivated, andthe interdendritic structure has a higher Zn concentration than thedendritic structure. Therefore, corrosion concentrates on theinterdendritic structure.

As a result, corrosion propagates through the interdendritic structurein a moth-eaten pattern, and corrosion propagation paths becomecomplicated, which makes it difficult for the corrosion to reach thesteel substrate (steel sheet).

For this reason, when a hot-dip Al—Zn coated steel sheet is used as anot-painted bare material, its corrosion resistance is superior to ahot-dip zinc-coated steel sheet having the same thickness of the coatinglayer.

When such a hot-dip Al—Zn coated steel sheet is used as an automobileouter panel, it is common that the coated steel sheet is supplied to anautomobile manufacturer, etc. in a state having been already coated in acontinuous hot-dip metal coating facility, processed into a panel partshape there, and thereafter the integral painting for an automobileincluding chemical conversion, electropainting, intermediate painting,and top coat painting is conducted.

It has been also studied to add Mg to a Zn—Al coating layer for thepurpose of improving the corrosion resistance. For example, PatentLiterature 3 discloses a hot-dip Zn—Al—Mg coated steel sheet in whichthe corrosion resistance is improved by forming a Zn/Al/MgZn₂ ternaryeutectic structure containing a Mg compound such as MgZn₂ in a coatinglayer. It is considered that the inclusion of Mg improves thesacrificial corrosion protection property of the coating layer toimprove the anticorrosion effect of the steel substrate.

Patent Literature 4 discloses a hot-dip Al—Zn coated steel sheet inwhich the corrosion resistance after painting is improved by breakingdown a passive state of the dendritic structure by inclusion of Sn orIn.

Further, Patent Literature 5 and Patent Literature 6 describe a hot-dipAl—Zn alloy coated steel sheet which contains Mg and Sn in combination.In Patent Literature 5 and 6, it is described that the hot-dip Al—Znalloy coated steel sheet is superior in corrosion resistance afterpainting and/or workability.

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2003-253416

Patent Literature 2: Japanese Patent Publication (JP-B) No. S46-7161

Patent Literature 3: JP-A No. 2001-329383

Patent Literature 4: Japan International Publication No. WO2014/155944

Patent Literature 5: JP-A No. 2015-214747

Patent Literature 6: JP-A No. 2002-180225

SUMMARY OF INVENTION Technical Problem

However, when the outer panel using the hot-dip Al—Zn coated steel sheetdescribed in Patent Literature 2 is damaged at the paint film and thecoating layer (when the steel substrate is exposed), the exposed steelsubstrate acts as a cathode, and preferential dissolution of Zn(selective corrosion of the interdendritic structure) occurs at theinterface between the paint film and the coating layer due to a uniquephase structure of the coating layer consisting of the above describedtwo structures of dendritic structure and interdendritic structure. Thispropagates deep into the sound area of the painting to cause a largepaint film blister. As a result, there arises a well-known problem thatthe erosion of the steel substrate cannot be suppressed.

Further, the interdendritic structure has lower hardness than thedendritic structure. Therefore, due to the difference in the hardnessbetween the interdendritic structure and the dendritic structure,deformation is concentrated in the interdendritic structure during pressworking. As a result, it is known that a crack is developed in thecoating layer to reach the steel substrate. Since corrosion is promotedin the vicinity of a crack where the steel substrate is exposed, in thecase of the conventional hot-dip Al—Zn coated steel sheet, not onlypaint film blistering occurs, but also the erosion of the steelsubstrate cannot be suppressed.

Further, a MgZn₂ phase included in the coating layer of the hot-dipZn—Al—Mg coated steel sheet described in Patent Literature 3 is brittle.Therefore, when the coated steel sheet is subjected to processing, thereis a risk that a large number of cracks may be generated starting fromthe Zn/Al/MgZn₂ ternary eutectic structure. Since the steel substrate isexposed when cracks are generated, there has been also a problem thatthe erosion of the steel substrate cannot be sufficiently suppressednear the processed area.

In the case of the hot-dip Al—Zn coated steel sheet described in PatentLiterature 4, Mg is not contained in the coating layer, and any attemptto reduce the corrosion rate of the coating layer itself has not beenmade. Therefore, it is conceivable that the sacrificial corrosionprotection property is not satisfactory as an automobile coated steelsheet, from the viewpoint of suppressing the erosion of the steelsubstrate for a long time period.

Further, with respect to the hot-dip Al—Zn alloy coated steel sheetdescribed in Patent Literature 5 and 6, control of the coating structurehas not been sufficiently studied, and therefore it is presumed thatMgZn₂ is formed as a brittle Mg-containing intermetallic compound in thecoating layer.

In this case, it is surmised that the resulting hot-dip Al—Zn alloycoated steel sheet is inferior in workability, and further that thesacrificial corrosion protection property is not sufficient, and a crackis formed in the coating layer at the time of pressing. Therefore, it ispredicted that corrosion propagates in the processed area starting fromthe crack.

It is considered that these hot-dip Al—Zn alloy coated steel sheets donot have workability or sacrificial corrosion protection propertysatisfactory as a coated steel sheet for an automobile, from theviewpoint of suppressing the erosion of the steel substrate for a longtime period.

That is, a hot-dip Zn coated steel sheet that is superior in bothcorrosion resistance after painting and sacrificial corrosion protectionproperty has heretofore not been developed, and in particular a coatedsteel sheet suitable for automotive applications has not been inexistence.

An object of an aspect of the present disclosure is to provide a coatedsteel sheet excellent in any of corrosion resistance after painting,sacrificial corrosion protection property, and workability.

Solution to Problem

The method of attaining the object includes the following aspect.

<1> A coated steel sheet including a steel sheet and a coating layerprovided on at least a part of a surface of the steel sheet, wherein:

the coating layer has a chemical composition including in terms of % bymass:

Al: from 15% to 60%

Mg: from 0.5% to 8.0%

Sn: from 0.5% to 20.0%

Si: from 0.05% to 1.50%

Bi: from 0% to 5.0%,

In: from 0% to 2.0%,

Ca: from 0% to 3.0%,

Y: from 0% to 0.5%,

La: from 0% to 0.5%,

Ce: from 0% to 0.5%,

Cr: from 0% to 0.25%,

Ti: from 0% to 0.25%,

Ni: from 0% to 0.25%,

Co: from 0% to 0.25%,

V: from 0% to 0.25%,

Nb: from 0% to 0.25%,

Cu: from 0% to 0.25%,

Mn: from 0% to 0.25%,

Sr: from 0% to 0.5%,

Sb: from 0% to 0.5%,

Pb: from 0% to 0.5%,

B: from 0% to 0.5%, and

a balance consisting of Zn and impurities, wherein:

the coating layer has a laminar Mg₂Sn phase-containing structure in anarea fraction of from 5 to 65%, and a structure containing a solidsolution of Zn and Al, and

the laminar Mg₂Sn phase-containing structure is a structure constitutedwith a Zn phase and a laminar Mg₂Sn phase having a thickness of lessthan 1 μm, and wherein the laminar Mg₂Sn phase exists dividing the Znphase into a plurality of regions.

<2> The coated steel sheet according to <1> above, wherein a content ofMg is from 0.5% to 3.0%, and a content of Sn is from 1.0% to 7.5% interms of % by mass.

<3> The coated steel sheet according to <1> or <2> above, wherein acontent of Al is from 20% to 60%, a content of Mg is from 1.0% to 2.0%,a content of Sn is from 1.0% to 5.0%, and a content of Si is from 0.05%to 1.0% in terms of % by mass.

<4> The coated steel sheet according to any one of <1> to <3> above,wherein the content of Sn and the content of Mg satisfy the followingFormula (1):Mg≤Sn≤2.5×Mg  Formula (1)

wherein, in Formula (1), each atomic symbol indicates a content of theelement in terms of % by mass.

<5> The coated steel sheet according to any one of <1> to <4> above,wherein the area fraction of the laminar Mg₂Sn phase-containingstructure is from 20% to 60%.

<6> The coated steel sheet according to any one of <1> to <5> above,wherein the area fraction of the laminar Mg₂Sn phase-containingstructure is from 30% to 60%.

<7> The coated steel sheet according to any one of <1> to <6> above,wherein an area fraction of the structure containing a solid solution ofZn and Al is from 35% to 95%.

<8> The coated steel sheet according to any one of <1> to <7> above,wherein the coating layer has a massive MgZn₂ phase with an equivalentcircle diameter of 1 μm or more in an area fraction of from 0% to 20%.

<9> The coated steel sheet according to any one of <1> to <8> above,wherein the coating layer has a massive MgZn₂ phase with an equivalentcircle diameter of 1 μm or more in an area fraction of from 0% to 5%.

<10> The coated steel sheet according to any one of <1> to <9> above,wherein the coating layer has a massive Zn phase with an equivalentcircle diameter of 2 μm or more in an area fraction of from 0% to 20%.

<11> The coated steel sheet according to any one of <1> to <10> above,wherein the coating layer has a massive Zn phase with an equivalentcircle diameter of 2 μm or more in an area fraction of from 0% to 10%.

<12> The coated steel sheet according to any one of <1> to <11> abovefurther including an interfacial alloy layer with a thickness of from100 nm to 1.5 μm consisting of an Al—Fe intermetallic compound betweenthe steel sheet and the coating layer.

Advantageous Effects of Invention

In an aspect of the present disclosure, a coated steel sheet excellentin any of corrosion resistance after painting, sacrificial corrosionprotection property, and workability can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a backscattered electron image (BSE image) taken with an SEMat a magnification of 2000× on an example of the coating layer of thecoated steel sheet according to the present disclosure (No. 26 ofExample).

FIG. 2 is a backscattered electron image (BSE image) taken with an SEMat a magnification of 6000× of the region A in FIG. 1.

FIG. 3 is a backscattered electron image (BSE image) taken with an SEMat a magnification of 2000× on the coating layer of the coated steelsheet of No. 24 of Example.

FIG. 4 is a backscattered electron image (BSE image) taken with an SEMat a magnification of 2000× on the coating layer of the coated steelsheet of No. 29 of Example.

FIG. 5 is a backscattered electron image (BSE image) with an SEM of across section of a coating layer for explaining a method of identifyinga Zn/Al/MgZn₂ ternary eutectic structure and measuring the area fractionof the same.

DESCRIPTION OF EMBODIMENTS

An example of the present disclosure will be described below.

In the present disclosure, the indication of “%” with respect to thecontent of each element in a chemical composition means “% by mass”.

A numerical range expressed by “x to y” includes the values of x and yin the range as the minimum and maximum values, respectively.

In a case “less than” or “more than” is affixed to the above x or y,such x or y is not included in the range as the minimum or maximumvalue.

The content of an element of a chemical composition may be expressed asthe amount of an element (for example, the amount of Zn, the amount ofMg, or the like), or the concentration of an element (for example, theconcentration of Zn, the concentration of Mg, or the like).

The “planar part corrosion resistance” refers to the corrosion resistantproperty of the coating layer itself.

The “sacrificial corrosion protection property” refers to a property ofsuppressing the corrosion of an area where a steel substrate is exposed(for example, a cut end surface of a coated steel sheet, an area wherecracking occurred during processing, or an area where a steel substratewas exposed by detachment of the coating layer).

The “equivalent circle diameter” is a diameter of a circle having thesame area as the region defined by the outline of a phase, which isidentified in a cross section of a coating layer (a cross section cut inthe thickness direction of the coating layer).

“C direction” means the direction perpendicular to the rolling directionof a steel sheet.

“L direction” means the direction parallel to the rolling direction of asteel sheet.

The coated steel sheet of the present disclosure includes a steel sheetand a coating layer provided on at least part of the surface of thesteel sheet.

The coating layer has a predetermined chemical composition. In addition,the coating layer has a laminar Mg₂Sn phase-containing structure in anarea fraction of from 5 to 65%, and a structure containing a solidsolution of Zn and Al (hereinafter, for convenience, also referred to as“dendritic structure).

The laminar Mg₂Sn phase-containing structure is a structure constitutedwith a Zn phase and a laminar Mg₂Sn phase having a thickness of lessthan 1 μm, wherein the laminar Mg₂Sn phase exists dividing the Zn phaseinto a plurality of regions.

The coated steel sheet of the present disclosure can be a coated steelsheet that is superior in any of corrosion resistance after painting,sacrificial corrosion protection property, and workability by virtue ofthe above constitution. The coated steel sheet of the present disclosurewas invented based on the following findings.

The inventors investigated the corrosion resistance after painting, thesacrificial corrosion protection property, and the workability of acoating layer suitable for a coated steel sheet for automotiveapplications, building materials applications, etc. As a result, thefollowing findings were obtained.

Although the Mg-containing intermetallic compound constitutes a brittlephase, a Mg₂Sn phase has more favorable plastic deformability ascompared to a MgZn₂ phase. By forming a structure in which the Mg₂Snphase exists dividing the Zn phase into a plurality of laminar regionsin a Zn phase having favorable plastic deformability, the laminar Mg₂Snphase-containing structure as a whole expresses favorable plasticdeformability, which contributes to improvement of the workability.

In addition, the Mg₂Sn phase serves as a supply source of Mg ions in acorrosive environment, and since Mg ions make the corrosion product toan insulating film, corrosion under the paint film in a painted statemay be suppressed. Although the mechanism is not clear, in the case of astructure in which the laminar Mg₂Sn phase exists dividing the Zn phaseinto a plurality of regions, the corrosion propagates along the laminarMg₂Sn phase, and as a result the laminar Mg₂Sn phase functions as asource of Mg ions for a long time period. The Mg₂Sn phase iselectrically less noble as compared to the MgZn₂ phase, and isinherently superior in sacrificial corrosion protection property.Therefore, it is presumed that it has an improving effect of corrosionresistance after painting and sacrificial corrosion protection property.

Therefore, when a laminar Mg₂Sn phase-containing structure constitutedwith a Zn phase and a laminar Mg₂Sn phase having a thickness of lessthan 1 μm, in which the laminar Mg₂Sn phase exists dividing the Zn phaseinto a plurality of regions, is made to be present at a predeterminedamount in terms of area fraction, the corrosion resistance afterpainting, the sacrificial corrosion protection property, and theworkability are all enhanced. Specifically, when the area fraction ofthe laminar Mg₂Sn phase-containing structure is 5% or more, thecorrosion resistance after painting, the sacrificial corrosionprotection property, and the workability become superior to acommercially available coated steel sheet.

From the above findings, it has been found that the coated steel sheetof the present disclosure is a coated steel sheet superior in any ofcorrosion resistance after painting, sacrificial corrosion protectionproperty, and workability.

In addition, since the coated steel sheet of the present disclosure hasa granular layer-dispersed structure that exhibits plastic deformabilitybe present in the coating layer, it can be also superior in resistanceto chipping and attain extension of the lifetime of a coated steel sheetafter painting.

The coated steel sheet of the present disclosure contains apredetermined amount of Al in the coating layer, and has a dendriticstructure that raises the melting point of the coating layer. Therefore,it can be also superior in resistance to seizure, and can prevent thecoating layer from sticking to a press mold during press molding. Thatis, it is possible for the coated steel sheet of the present disclosureto be superior in both corrosion resistance after painting and pressformability.

The coated steel sheet of the present disclosure will be described indetail below.

First, a steel sheet will be described.

There is no particular restriction on a steel sheet used as an originalsheet for coating, and various steel sheets of an Al-killed steel, anultra-low carbon steel, a high carbon steel, various high tensilestrength steels, a Ni, Cr-containing steel, or the like can be used.There is no particular restriction on a steelmaking method, the strengthof a steel, or a pretreatment of steel sheet, such as a hot rollingmethod, a pickling method, or a cold rolling method.

There is also no particular restriction on the chemical composition (C,Si, etc.) of a steel sheet. It has not been confirmed that elements suchas Ni, Mn, Cr, Mo, Ti, or B contained in the steel sheet affect thecoating layer.

Next, a coating layer will be described.

First, the chemical composition of a coating layer will be described.

The chemical composition of a coated steel sheet includes Al, Mg, Sn,and Si as essential elements, and the balance is Zn and impurities. Thechemical composition of a coated steel sheet may include at least one ofBi, In, Ca, Y, La, Ce, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb and Bas an optional element. That is, the optional element needs not becontained.

The content of each element in the coating layer means the averagecontent of each element contained in the entire coating layer.

[Al: From 15% to 60%]

Al is an essential element for improving the corrosion resistance afterpainting and the seizure resistance of a coating layer. Most of Al ispresent as an Al phase inside a dendritic structure in the coatinglayer.

The dendritic structure described later is not passivated due to theeffect of Sn contained, and in a state not to become a factor fordecreasing the corrosion resistance after painting. Meanwhile, when themelting point of the coating layer is low, there arises a problem ofseizure of a metal in the coating layer with a press mold. However, asthe Al concentration becomes higher, the area fraction of the dendriticstructure that is a high melting point structure increases, and, as aresult, it is possible to prevent the coating layer from sticking to thepress mold at the time of press molding (namely, the resistance toseizure is improved).

The Al concentration required to secure the area fraction of thedendritic structure capable of developing sufficient resistance toseizure is 15% or more. Therefore, the lower limit of the Alconcentration is set at 15%. A preferable Al concentration is 20% ormore.

Meanwhile, when the Al concentration exceeds 60%, an “interfacial alloylayer composed of an Al—Fe intermetallic compound” formed at theinterface between a coating layer and a steel substrate described latergrows excessively to impair the workability. Therefore, the upper limitof the Al concentration is set at 60%. A preferable Al concentration is40% or less.

[Mg: From 0.5% to 8.0%]

Mg is an essential element for forming a laminar Mg₂Sn phase-containingstructure in a coating layer to impart favorable corrosion resistanceafter painting, sacrificial corrosion protection property, andworkability to the coating layer. Mg is present in the coating layer ina form of a Mg-containing intermetallic compound, and dissolved into acorrosive environment as Mg ions under a corrosive environment. Mg ionsconvert a Zn-based corrosion product to an insulating film, and rust toa barrier film. From the above, penetration of a corrosion factor intothe coating layer and under the paint film can be suppressed so as tocontribute to improvement of the corrosion resistance after painting.Most of Mg is contained in a laminar Mg₂Sn phase-containing structure.The formation of the granular Mg₂Sn phase-containing structure improvesany of the corrosion resistance after painting, the sacrificialcorrosion protection property, and the workability. The Mg concentrationrequired to improve the corrosion resistance after painting, thesacrificial corrosion protection property, and the workability is 0.5%.Therefore, the lower limit of the Mg concentration is set at 0.5%. Apreferable Mg concentration is 1.0% or more.

Meanwhile, when the Mg concentration exceeds 8.0%, a massive MgZn₂ phasedescribed later is excessively generated to impair the workability.Therefore, the upper limit of the Mg concentration is set at 8.0%. Fromthe viewpoint of suppressing formation of the massive MgZn₂ phase thatimpairs the workability, a preferable Mg concentration is 3.0% or less.A more preferable Mg concentration is 2.0% or less.

[Sn: From 0.5% to 20.0%]

Sn is an essential element for forming a laminar Mg₂Sn phase-containingstructure in a coating layer together with Mg to impart favorablecorrosion resistance after painting, sacrificial corrosion protectionproperty, and workability to the coating layer. Further, Sn is anelement having an effect of suppressing formation of a massive MgZn₂phase together with a Zn/Al/MgZn₂ ternary eutectic structure.

Therefore, Sn is also an element that enhances the corrosion resistanceafter painting, the sacrificial corrosion protection property, and theworkability of the coating layer.

When the Sn concentration is low, it becomes difficult to form a laminarMg₂Sn phase-containing structure, while the generation amounts of aZn/Al/MgZn₂ ternary eutectic structure and a massive MgZn₂ phaseincrease. As a result, the corrosion resistance after painting, thesacrificial corrosion protection property, and the workability of thecoating layer tend to decrease. Therefore, the lower limit of the Snconcentration is set at 0.5%. From the viewpoint of sufficiently forminga laminar Mg₂Sn phase-containing structure and sufficiently suppressingformation of a Zn/Al/MgZn₂ ternary eutectic structure and a massiveMgZn₂ phase, a preferable Sn concentration is 0.1% or more, and a morepreferable Sn concentration is 1.5% or more.

Meanwhile, when the Sn concentration is excessive, the surplus Sncrystallizes as a potentially nobler Sn phase to decrease the corrosionresistance after painting, and the sacrificial corrosion protectionproperty. Therefore, the upper limit of the Sn concentration is set at20.0%. From the viewpoint of improving the corrosion resistance afterpainting, a preferable Sn concentration is 7.5% or less, and a morepreferable Sn concentration is 5.0% or less.

[Si: From 0.05% to 1.50%]

Si, when contained in a coating bath, is an element that suppresses thereactivity of Zn and Al contained in the coating bath with Fe element inan original sheet for coating. That is, Si is an essential element tocontrol the formation behavior of an interfacial alloy layer composed ofan Al—Fe intermetallic compound having an effect on the adhesion andworkability of the coating layer (in particular, an interfacial alloylayer containing or consisting of Fe₂Al₅) by controlling the reactivitybetween the coating layer and the steel substrate. The minimum Siconcentration necessary for suppressing the interfacial alloy layer is0.05%.

When the Si concentration is less than 0.05%, an interfacial alloy layergrows immediately after dipping an original sheet for coating in thecoating bath to make it difficult for the coating layer to acquirefavorable ductility, and therefore the workability tends to decrease.Consequently, the lower limit of the Si concentration is set at 0.05%. Apreferable Si concentration is 0.2% or more.

Meanwhile, when the Si concentration exceeds 1.50%, a potentially noblerSi phase remains in the coating layer, and functions as a cathode zonein corrosion. As a result, it leads to decrease in corrosion resistanceafter painting. Therefore, the upper limit of the Si concentration isset at 1.50%. A preferable Si concentration is 1.0% or less.

In this regard, Si may occasionally exist in the coating layer as aMg₂Si phase which is an intermetallic compound with Mg, but insofar asthe area fraction of the Mg₂Si phase is 5% or less, it does not affectthe performance at all.

[Bi: From 0% to 5.0%]

Bi is an element contributing to improvement of the sacrificialcorrosion protection property. Therefore, the lower limit of the Biconcentration should be more than 0% (preferably 0.1% or more, and morepreferably 3.0 or more).

Meanwhile, when the Bi concentration increases, the coating layer iseasily corroded under the paint film, and the corrosion resistance afterpainting tends to deteriorate in the sense that the paint filmblistering is prone to become large. Therefore, the upper limit of theBi concentration is set at 5.0% or less (preferably 0.5% or less, andmore preferably 0.1% or less).

[In: From 0% to 2.0%]

In is an element contributing to improvement of the sacrificialcorrosion protection property. Therefore, the lower limit of the Inshould be more than 0% (preferably 0.1% or more, and more preferably 3.0or more).

Meanwhile, when the In concentration increases, the coating layer iseasily corroded under the paint film, and the corrosion resistance afterpainting tends to deteriorate in the sense that the paint filmblistering is prone to become large. Therefore, the upper limit of theIn concentration is set at 2.0% or less (preferably 0.3% or less).

[Ca: From 0% to 3.0%]

Ca is an element capable of adjusting the Mg dissolution amount optimumfor imparting corrosion resistance after painting and sacrificialcorrosion protection property. Therefore, the lower limit of the Caconcentration should be more than 0% (preferably 0.05% or more).

Meanwhile, when the Ca concentration increases, the workability tends todeteriorate. Therefore, the upper limit of the Ca concentration is setat 3.0% or less (preferably 1.0% or less).

[Y: From 0% to 0.5%]

Y is an element contributing to sacrificial corrosion protectionproperty. Therefore, the lower limit of the Y concentration should bemore than 0% (preferably 0.1% or more).

Meanwhile, when the Y concentration increases, the corrosion resistanceafter painting tends to deteriorate. Therefore, the upper limit of the Yconcentration is set at 0.5% or less (preferably 0.3% or less).

[La and Ce: From 0% to 0.5%]

La and Ce are elements contributing to sacrificial corrosion protectionproperty. Therefore, the lower limits of the La concentration and the Ceconcentration should be respectively more than 0% (preferably 0.1% ormore).

Meanwhile, when the La concentration and the Ce concentration increase,the corrosion resistance after painting tends to deteriorate. Therefore,the upper limits of the La concentration and the Ce concentration arerespectively set at 0.5% or less (preferably 0.3% or less).

[Cr, Ti, Ni, Co, V, Nb, Cu, and Mn: From 0% to 0.25%]

Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are elements contributing tosacrificial corrosion protection property. Therefore, the lower limitsof the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn should berespectively more than 0% (preferably 0.05% or more, and more preferably0.1% or more).

Meanwhile, when the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mnincrease, the corrosion resistance after painting tends to deteriorate.Therefore, the upper limits of the concentration of Cr, Ti, Ni, Co, V,Nb, Cu, and Mn are respectively set at 0.25% or less.

[Sr, Sb, Pb, and B: From 0% to 0.5%]

Sr, Sb, Pb, and B are elements contributing to sacrificial corrosionprotection property. Therefore, the lower limits of the concentrationsof Sr, Sb, Pb, and B should be respectively more than 0 (preferably0.05% or more, and more preferably 0.1% or more).

Meanwhile, when the concentrations of Sr, Sb, Pb, and B increase, thecorrosion resistance after painting tends to deteriorate. Therefore, theupper limits of the concentrations of Sr, Sb, Pb, and B are respectivelyset at 0.5% or less (preferably 0.1% or less).

[The Balance: Zn and Impurities]

The balance of the chemical composition of the coating layer is Zn andimpurities.

Zn is contained in the coating layer at a certain concentration or morein order to properly secure the sacrificial protection performance ofthe coating layer, the planar part corrosion resistance, and thepainting substrate treatment property. From these viewpoints, thechemical composition of the coating layer is mostly occupied by Al andZn.

The impurities refer to components included in a raw material orcomponents entered in the production process, which are notintentionally added. For example, a small amount of a component such asFe may be mixed in into the coating layer as an impurity due to mutualatomic diffusion between the steel substrate (steel sheet) and thecoating bath.

For example, when a coating layer is formed by a hot-dip metal coatingmethod, the coating layer may contain Fe at a certain concentration asan impurity. It has been confirmed that the performance is not adverselyaffected by Fe up to a concentration in the coating layer of 3.0%.

[Preferred Chemical Composition of Coating Layer]

In the chemical composition of a coating layer, the content of Mg ispreferably from 0.5% to 3.0%, and the content of Sn is preferably from1.0% to 7.5%. When the Mg concentration and Sn concentration are in theabove ranges, the corrosion resistance after painting, the sacrificialcorrosion protection property, and the workability are further improved.

In particular, it is preferable that the content of Al is from 20% to60%, the content of Mg is from 1.0% to 2.0%, the content of Sn is from1.0% to 5.0%, and the content of Si is from 0.05% to 1.0% in thechemical composition of a coating layer. When the Al concentration, Mgconcentration, Sn concentration, and Si concentration are in the aboveranges, the corrosion resistance after painting, the sacrificialcorrosion protection property, and the workability are further improved.In addition, the resistance to seizure is also further improved.[Mg % by mass≤Sn % by mass≤2.5×Mg % by mass]  Formula (1):

In order to further improve the corrosion resistance after painting, thesacrificial corrosion protection property, and the workability, it ispreferable that a granular Mg₂Sn phase-containing structure issufficiently formed, and the formation of a Zn/Al/MgZn₂ ternary eutecticstructure and a massive MgZn₂ phase is sufficiently suppressed.

For this purpose, the content of Sn and the content of Mg preferablysatisfy the following Formula (1), and more preferably satisfy thefollowing Formula (2).Mg≤Sn≤2.5×Mg  Formula (1)1.5×Mg≤Sn≤2.0×Mg  Formula (2)

In Formula (1) and Formula (2), each atomic symbol indicates the contentof the element in terms of % by mass.

When the Sn concentration does not satisfy Formula (1) and there is ashortage of Sn with respect to Mg, a massive MgZn₂ phase is formed, andthe corrosion resistance after painting, and the sacrificial corrosionprotection property tend to decrease together with the workability.

Meanwhile, when the Sn concentration does not satisfy Formula (1) andthere is a surplus of Sn with respect to Mg, a potentially nobler Snphase crystallizes, and the corrosion resistance after painting, and thesacrificial corrosion protection property tend to decrease.

Next, the metallographic structure of a coating layer will be described.

The coating layer has a laminar Mg₂Sn phase-containing structure and adendritic structure (structure containing a solid solution of Zn andAl).

And, the coating layer has in some cases a massive MgZn₂ phase with anequivalent circle diameter of 1 μm or more, a massive Zn phase with anequivalent circle diameter of 2 μm or more, a Zn/Al/MgZn₂ ternaryeutectic structure, or the like as a structure other than the granularMg₂Sn phase-containing structure

Here, a backscattered electron image (BSE image) of an example of thecoating layer of the coated steel sheet of the present disclosure takenwith an SEM at a magnification of 2000× is shown.

As shown in FIG. 1, the coated steel sheet has, for example, a coatinglayer 1, a steel sheet 2, and an interfacial alloy layer 3 composed ofan Al—Fe intermetallic compound between the coating layer 1 and thesteel sheet 2.

The structure of the coating layer 1 is mainly constituted with alaminar Mg₂Sn phase-containing structure 4, and a dendritic structure 5.Further, as shown in FIG. 2, which is an enlarged view of the region Ain FIG. 1, the laminar Mg₂Sn phase-containing structure 4 has astructure in which a laminar Mg₂Sn phase 7 with a thickness of less than1 μm exists in the Zn phase 6 dividing the Zn phase 6 into a pluralityof regions.

In FIG. 1, the dendritic structure 5 corresponds to the gray-coloredregion as well as the black-colored region surrounded by the formerregion. The difference in color between the two regions is due to thedifference in the Al concentrations. Specifically, the dendriticstructure 5 with a low Al concentration is the gray-colored region, andthe gray-colored dendritic structure 5 with a high Al concentration isthe black-colored region.

In addition to the laminar Mg₂Sn phase-containing structure 4 and thedendritic structure 5, the coating layer 1 may occasionally contain amassive MgZn₂ phase 10 (see FIG. 3), a massive Zn phase 9 (see FIG. 3),and a Zn/Al/MgZn₂ ternary eutectic structure 8 (see FIG. 3).

[Laminar Mg₂Sn Phase-Containing Structure: Area Fraction from 5 to 65%]

The laminar Mg₂Sn phase-containing structure includes a Zn phase and alaminar Mg₂Sn phase with a thickness of less than 1 μm, and the laminarMg₂Sn phase exists dividing the Zn phase into a plurality of regions.

When a cross section or a surface of the coating layer is observed, thelaminar Mg₂Sn phase-containing structure is, for example, a structurepresent in the interstices of the dendritic Zn phase. More specifically,it is a structure in which a laminar Mg₂Sn phase having a thickness ofless than 1 μm exists in the Zn phase present in the interstices of thedendritic Zn phase structure dividing the Zn phase into a plurality ofregions, when a cross section or a surface of the coating layer isobserved.

In this regard, a laminar Mg₂Sn phase is a Mg₂Sn phase present in theinterstices between the adjacent Zn phase structures branched in adendritic shape. The laminar Mg₂Sn phase becomes a layer having athickness of less than 1 μm due to the close positional relationshipbetween the adjacent dendritic Zn phase structures. The laminar Mg₂Snphase is shaped to cover each of dendritic Zn phase structures, and as aresult, the Zn phase branched in a dendritic shape is divided into aplurality of regions.

Further, it is essential for attaining the object of the presentdisclosure that the thickness of a laminar Mg₂Sn phase is less than 1μm. The more finely the dendritic Zn phase structure is divided, thethinner the thickness of the laminar Mg₂Sn phase becomes. Insofar as thethickness of the laminar Mg₂Sn phase is less than 1 μm, the laminarMg₂Sn phase-containing structure can sufficiently exhibit plasticdeformability. Although there is no particular restriction on the lowerlimit of the thickness of the laminar Mg₂Sn phase, it is, for example,10 nm or more.

In addition, under the cooling conditions of the production methoddescribed later, the area fraction of the Mg₂Sn phase in the laminarMg₂Sn phase-containing structure becomes 10% or more. In this case, itis possible to improve the corrosion resistance while maintaining theplastic deformability. On the other hand, when the cooling is performedoutside the appropriate cooling conditions, the area fraction of theMg₂Sn phase becomes less than 10%, and a plate-like Mg₂Sn phase isformed not in the interstices between the adjacent dendritic Zn phasestructures, rather in a mixed form with coarse Zn phases. In this case,the amount of the Mg₂Sn phase decreases, and as a result adequatecorrosion resistance can be hardly obtained. Therefore, when the coolingwas not performed under proper cooling conditions, and the area fractionof the Mg₂Sn phase becomes less than 10%, the structure is referred toas a plate-like Mg₂Sn phase-containing structure (Zn phase+plate-likeMg₂Sn phase), and discriminated from a laminar Mg₂Sn phase-containingstructure.

In the plate-like Mg₂Sn phase-containing structure in which a dendriticZn phase structure becomes coarse, a stress is prone to concentrate onthe small amount of Mg₂Sn phase, and therefore such structure isinferior to a laminar Mg₂Sn phase-containing structure in plasticdeformability.

The reason for the above is presumed as follows. The laminar Mg₂Snphases are provided so as to cover each of fine dendritic Zn structures.In contrast, the plate-like Mg₂Sn phases are present in a mixed statewith coarse dendritic Zn structures. Therefore, in the laminar Mg₂Snphase-containing structure, a stress applied to the laminar Mg₂Sn phaseis easily dispersed, while in the plate-like Mg₂Sn phase-containingstructure, a stress applied to the plate-like Mg₂Sn phase is prone to beconcentrated. Therefore, the plate-like Mg₂Sn phase-containing structureis presumably inferior to the laminar Mg₂Sn phase-containing structurein plastic deformability.

While not wishing to be bound by theory, it is believed that the laminarMg₂Sn phase-containing structure is a structure formed by allowing a Znphase to grow rapidly in a dendritic form, and a Mg₂Sn phase to solidifyin a laminar form between adjacent branches of a dendritic Zn phase atthe time of final solidification in a production process of a coatinglayer. In fact, in the coating layer according to this disclosure, asshown in the region B of FIG. 2, a Zn phase 6 further grows in adendrite form in the interstices of the dendritic structures 5, andformation of a structure in which a laminar Mg₂Sn phase 7 is presentaround this Zn dendrite can be confirmed. When a cross-section or asurface of the coating layer is observed, such a structure isconceivably observed as a structure in which the laminar Mg₂Sn phase 7exists dividing the Zn phase 6 into a plurality of regions, as shown inFIGS. 1 and 2.

In other words, the laminar Mg₂Sn phase-containing structure is astructure constituted with dendritic Zn phases and laminar Mg₂Sn phaseswith a thickness of less than 1 μm existing between the branches of thedendritic Zn phase.

In this case, in the laminar Mg₂Sn phase-containing structure, the areafraction of the laminar Mg₂Sn phase with respect to the laminar Mg₂Snphase-containing structure (that is, the Zn phase and the laminar Mg₂Snphase) is preferably from 10 to 50%. Meanwhile, the average thickness ofthe laminar Mg₂Sn phase is preferably not less than 0.01 but less than 1μm.

There is no particular restriction on the average composition of theentire laminar Mg₂Sn phase-containing structure, and, for example, theMg concentration is from 1 to 10% by mass, the Sn concentration is from1 to 25% by mass, and the Al concentration is from 1 to 8% by mass,while the balance consists of Zn, and impurities of less than about 2%by mass. The composition of the entire laminar Mg₂Sn phase-containingstructure may also include the above-mentioned optional elements thatcan be included in the chemical composition of the coating layer.

In this regard, in the present disclosure, a granular phase of anintermetallic compound corresponding to any of the following (1) to (5)is also regarded as the laminar Mg₂Sn phase.

(1) Mg₂Sn in which an element such as Si is interstitially dissolved;

(2) Mg₉Sn₅ formed through transformation of a Mg₂Sn phase;

(3) Substituted Mg₂Sn and Mg₉Sn₅ (substitution product of Mg₂Sn andMg₉Sn₅) in which at least one of Bi, In, Cr, Ti, Ni, Co, V, Nb, Cu, andMn is substituted for part of Sn;

(4) Substituted Mg₂Sn and Mg₉Sn₅ (substitution product of Mg₂Sn andMg₉Sn₅) in which at least one of Ca, Y, La and Ce is substituted forpart of Mg;

(5) Substituted Mg₂Sn and Mg₉Sn₅ (substitution product of Mg₂Sn andMg₉Sn₅) in which at least one of Ca, Y, La, and Ce is substituted forpart of Mg, and at least one of Bi, In, Cr, Ti, Ni, Co, V, Nb, Cu, andMn is substituted for part of Sn.

A laminar Mg₂Sn phase-containing structure has an effect of improvingthe corrosion resistance after painting, sacrificial corrosionprotection property, and workability.

Although it includes a Mg₂Sn phase which is a brittle Mg-containingintermetallic compound as described above, the Mg₂Sn phase has higherplastic deformability compared to a MgZn₂ phase.

Although the Mg-containing intermetallic compound is a brittle phase asdescribed above, the Mg₂Sn phase has a higher plastic deformabilitycompared to the MgZn₂ phase. When a structure is so constructed that theMg₂Sn phase exists in a Zn phase with favorable plastic deformabilitydividing the Zn phase into a plurality of laminar regions, the structureas a whole expresses favorable plastic deformability to contribute toimprovement of the workability.

In addition, the Mg₂Sn phase serves as a supply source of Mg ions in acorrosive environment, and the Mg ions make the corrosion product to aninsulating film, so that corrosion under the paint film in a paintedstate may be suppressed. Although the details of the mechanism are notclear, in the case of a structure in which the laminar Mg₂Sn phaseexists dividing the Zn phase into a plurality of regions, the corrosionpropagates along the laminar Mg₂Sn phase, and as a result the laminarMg₂Sn phase functions as a source of Mg ions for a long time period. TheMg₂Sn phase is electrically less noble as compared to the MgZn₂ phase,and is inherently superior in sacrificial corrosion protection property.Therefore, it is presumed that it has the improvement effect on thecorrosion resistance after painting and the sacrificial corrosionprotection property.

The higher the area fraction of the laminar Mg₂Sn phase-containingstructure present in the coating layer is, the higher the improvementeffect on the corrosion resistance after painting, sacrificial corrosionprotection property, and workability with the laminar Mg₂Snphase-containing structure becomes.

When the area fraction of the laminar Mg₂Sn phase-containing structureis less than 5%, the improvement effect on the corrosion resistanceafter painting, the sacrificial corrosion protection property, and theworkability cannot be obtained. Therefore, the lower limit of the areafraction of the laminar Mg₂Sn phase-containing structure is set at 5%.From the viewpoint of reliably improving both the corrosion resistanceafter painting, the sacrificial corrosion protection property, and theworkability, the area fraction of the laminar Mg₂Sn phase-containingstructure is preferably 20% or more, and more preferably 30% or more.

Meanwhile, as described above, the higher the area fraction of thelaminar Mg₂Sn phase-containing structure becomes, the greater the effectof improving the corrosion resistance after painting, the sacrificialcorrosion protection property, and the workability becomes. Althoughthere is no particular restriction on the upper limit value from theviewpoint of performance, the producible area fraction of the laminarMg₂Sn phase-containing structure is 65% at the maximum due to productionrestrictions. Therefore, the upper limit of the area fraction of thelaminar Mg₂Sn phase-containing structure is set at 65%. From theviewpoint of stable production, the area fraction of the laminar Mg₂Snphase-containing structure is preferably 60% or less.

That is, the area fraction of the laminar Mg₂Sn phase-containingstructure is from 5 to 65%. The area fraction of the laminar Mg₂Snphase-containing structure is preferably from 20 to 60%, and morepreferably from 30 to 60%.

[Dendritic Structure: Area Fraction from 35% to 95%]

A dendritic structure is a structure containing a solid solution of Znand Al. Specifically, a dendritic structure is a structure finelyseparated to Al phases and Zn phases, and is a structure with the Alconcentration of from 15 to 85% and the Zn concentration of from 15 to85%. Therefore, a dendritic structure is a structure which hasfundamentally favorable plastic deformability, and can contribute toimprovement of the workability of the coating layer. In addition, it isalso a structure contributing to improvement of the seizure resistance.

In order to ensure excellent workability, the area fraction of thedendritic structure is preferably 35% or more. From the viewpoint ofimparting excellent workability to the coating layer, the area fractionof the dendritic structure is more preferably 40% or more. Meanwhile,from the viewpoint of production, the upper limit value with respect tothe dendritic structure is preferably 95%. From the viewpoint ofimproving the corrosion resistance after painting and the workability bythe granular phase dispersed phase, the dendritic structure occupiespreferably 80% or less, more preferably 70% or less.

That is, the area fraction of the dendritic structure is preferably from35 to 95%, more preferably from 35 or 40 to 80%, and further preferablyfrom 35 or 40 to 70%.

[Massive Zn Phase: Area Fraction from 0% to 20%]

A massive Zn phase is present in an irregular form in the coating layer,and is a massive Zn phase having an equivalent circle diameter of 2 μmor more. The upper limit of the equivalent circle diameter of themassive Zn phase is not particularly limited, but is, for example, 10 μmor less.

As the area fraction of the massive Zn phase increases, the resistanceto seizure and the corrosion resistance tend to decrease. Therefore,from the viewpoint of securing the resistance to seizure and thecorrosion resistance, the area fraction of the massive Zn phase ispreferably 20% or less. From the viewpoint of securing sufficientresistance to seizure and corrosion resistance, the area fraction of themassive Zn phase is more preferably 10% or less. The area fraction ofthe massive Zn phase is most preferably 0% (namely, it is mostpreferable that a massive Zn phase is not included).

That is, the area fraction of the massive Zn phase is preferably from 0to 20%, more preferably from 0 to 10%, and further preferably 0%.

[Massive MgZn₂ Phase: Area Fraction from 0% to 20%]

A massive MgZn₂ phase is present in an irregular form in the coatinglayer and is a massive Zn phase having an equivalent circle diameter of2 μm or more. The upper limit of the equivalent circle diameter of amassive MgZn₂ phase is not particularly limited, but is, for example, 10μm or less.

The massive MgZn₂ phase is a brittle phase and tends to become astarting point of cracking at the time of processing. Corrosion may beaccelerated in the vicinity of the crack which may cause decrease incorrosion resistance after painting at a processed part. As the areafraction of the massive MgZn₂ phase increases, the corrosion resistanceafter painting and the workability tend to decrease. Therefore, from theviewpoint of securing corrosion resistance after painting andworkability, the area fraction of the massive MgZn₂ phase is preferably20% or less. From the viewpoint of securing sufficient corrosionresistance after painting and workability, the area fraction of themassive MgZn₂ phase is more preferably 5% or less. The area fraction ofthe massive MgZn₂ phase is most preferably 0% (namely, it is mostpreferable that a massive MgZn₂ phase is not included).

That is, the area fraction of the massive MgZn₂ phase is preferably from0 to 20%, more preferably from 0 to 5%, and further preferably 0%.

[Zn/Al/MgZn₂ Ternary Eutectic Structure: Area Fraction from 0% to 3%]

The Zn/Al/MgZn₂ ternary eutectic structure is a structure consisting ofan Al phase, a Zn phase, and a MgZn phase. Since the size variesdepending on the component composition, the shape of each phase isindeterminate. However, since in a eutectic structure, element movementduring solidification is suppressed due to transformation at a constanttemperature, the respective phases form an intricate pattern, andordinarily the respective phases finely precipitate (see FIG. 5).

Ordinarily the respective phases are configured such that the Zn phaseis large and forms islands, the MgZn phase is second largest and fillsthe gaps of the Zn phases, and the Al phase is dispersed in a spotpattern among the MgZn₂ phases. Although the constituent phases are notchanged by the component composition, the positional relationshipdepends on the component variation just before solidification and thereare a case where the MgZn₂ phase precipitates to form islands, and acase where the Al phase, or the MgZn₂ phase does so.

The method of identifying a ternary eutectic structure will be describedlater.

With respect to a Zn/Al/MgZn₂ ternary eutectic structure, the MgZn₂phase in the ternary eutectic structure, which is brittle andsusceptible to corrosion, tends to become a starting point of a crack atthe time of processing. Corrosion may be accelerated in the vicinity ofthe crack, which may cause decrease in the corrosion resistance afterpainting of a processed part. As the area fraction of the Zn/Al/MgZn₂ternary eutectic structure increases, the corrosion resistance afterpainting and the workability tend to decrease. Therefore, from theviewpoint of securing corrosion resistance after painting andworkability, the area fraction of the Zn/Al/MgZn₂ ternary eutecticstructure is preferably 3% or less. From the viewpoint of securingsufficient corrosion resistance after painting and workability, the areafraction of the Zn/Al/MgZn₂ ternary eutectic structure is mostpreferably 0% (namely, it is most preferable that the Zn/Al/MgZn₂ternary eutectic structure is not included).

That is, the area fraction of the Zn/Al/MgZn₂ ternary eutectic structureis preferably from 0 to 3%, and most preferably 0%.

The thickness of the coating layer is, for example, about 100 μm orless. Since the thickness of the entire coating layer depends on thecoating conditions, the upper limit and the lower limit of the thicknessof the entire coating layer are not particularly restricted. Forexample, the thickness of the entire coating layer is related to theviscosity and the specific gravity of the coating bath in theconventional hot-dip metal coating method. Further, the coating amountin terms of weight per unit area may be adjusted by the drawing speed ofthe steel sheet (original sheet for coating) and the intensity of thewiping. Therefore, the lower limit of the thickness of the entirecoating layer is, for example, about 2 Meanwhile, the thickness of thecoating layer, which can be produced by a hot-dip metal coating method,is about 95 μm due to the own weight and uniformity of the coatingmetal.

Therefore, the thickness of the coating layer is preferably from 2 to 95μm.

Next, an interfacial alloy layer will be described.

The coated steel sheet of the present disclosure may further have aninterfacial alloy layer composed of an Al—Fe intermetallic compoundbetween the steel sheet and the coating layer. Generally, an interfacialalloy layer composed of an Al—Fe intermetallic compound of 3 μm or lessis formed between the coating layer and the steel sheet. However, aninterfacial alloy layer is not necessarily formed depending on theformation conditions of a coating layer.

The interfacial alloy layer preferably has a thickness of 100 nm or morein order to ensure the adhesion between the steel substrate (steelsheet) and the coating layer. Meanwhile, since the Al—Fe intermetalliccompound composing the interfacial alloy layer is a brittleintermetallic compound, when the thickness of the interfacial alloylayer exceeds 1.5 μm, the resistance to chipping may be reduced.

Therefore, when the coated steel sheet of the present disclosure has aninterfacial alloy layer, the thickness of the interfacial alloy layer ispreferably from 100 nm to 1.5 μm.

Since the interfacial alloy layer is in a solid state in which Si isdissolved, it has a role of suppressing an alloying reaction between thecoating layer and the steel substrate.

In this regard, the interfacial alloy layer composed of the Al—Feintermetallic compound is a layer in which the Al₅Fe phase is the mainphase. The Al—Fe alloy layer is formed by mutual atomic diffusion of thesteel substrate (steel sheet) and the coating bath. However, theinterfacial alloy layer may partially contain only a small amount of anAlFe phase, an Al₃Fe phase, an Al₅Fe₂ phase, or the like.

The interfacial alloy layer may also contain various elements such as Znor Si, which are components of the coating layer. In particular, when Siis incorporated into the interfacial alloy layer, an Al—Fe—Siintermetallic compound is formed in the interfacial alloy layer.

Furthermore, when an original sheet for coating out of various kinds ofpre-coated steel sheet is used, the interfacial alloy layer may includea pre-coating component (for example, Ni). When the pre-coatingcomponent (for example, Ni) is incorporated into an interfacial alloylayer, an Al—Fe—Ni intermetallic compound is formed in the interfacialalloy layer.

That is, the interfacial alloy layer composed of an Al—Fe intermetalliccompound means a layer which includes the above-described various modesof alloy layers besides the alloy layer constituted mainly with an Al₅Fephase

An example of the method of producing the coated steel sheet of thepresent disclosure will be described below.

The coated steel of the present disclosure is obtained by forming acoating layer on the surface (that is, one side or both sides) of anoriginal sheet for coating by a hot-dip metal coating method.

As a method of producing a coated steel sheet according to the presentdisclosure, a Sendzimir method, a pre-coating method, or the like may beapplied. When Ni is used as a type of pre-coating, Ni may be containedin an “Interfacial alloy layer composed of an Al—Fe intermetalliccompound” which may be formed when the coating layer is heated.

A coating bath is formed by mixing a pure metal or an alloy into therange of the chemical composition of the above-mentioned coating layer,and melting the same in a range of 450 to 650° C. as an initial make-upof electrolytic bath.

Then, an original sheet for coating whose surface is sufficientlyreduced is immersed in the coating bath which is kept at a predeterminedbath temperature after the initial make-up of electrolytic bath, takenout, and then cooled down thereby completing a coating layer on thesurface of the original sheet for coating (steel sheet). For regulatingthe coating amount of the coating layer, for example, wiping with a N₂gas is performed immediately after the original sheet for coating istaken out from the coating bath.

In this regard, the cooling rate in the temperature range fromimmediately after removal of the original sheet for coating from thecoating bath (that is, the coating bath temperature) to 320° C. is setat 10° C./s or less, and the cooling rate in the temperature range offrom 320° C. to 280° C. is set at 20° C./s or more.

The backscattered electron image (BSE image) with an SEM of the crosssection of the coating layer of the coated steel sheet of the presentdisclosure shown in FIG. 1 is a backscattered electron image (BSE image)with an SEM of the cross section of the coating layer of the coatedsteel sheet prepared using the cooling rate of 10° C./s in thetemperature range from the temperature of the coating bath to 320° C.,and the cooling rate of 40° C./s in the temperature range of from 320°C. to 280° C.

As shown in FIG. 1, under the above-described cooling conditions, astructure having a laminar Mg₂Sn phase-containing structure 4 and adendritic structure 5 in the coating layer is formed.

The regulation of the cooling rates can be achieved by any method knownto those skilled in the art. For example, there is a method ofregulating the cooling rate by appropriately adjusting the flow rate ofa cooling gas. Particularly, when water cooling is utilized, anextremely high cooling rate even exceeding 100° C./s can be alsorealized.

Meanwhile, even when the cooling rate in the temperature range fromimmediately after removal of the original sheet for coating from thecoating bath (that is, the coating bath temperature) to 320° C. is 10°C./s or less, and the cooling rate in the temperature range of from 320°C. to 280° C. is 20° C./s or more, unless the Sn concentration isappropriate, a sufficient amount of the laminar Mg₂Sn phase-containingstructure 4 is not necessarily formed. For example, as shown in FIG. 3,when Sn is not added, a laminar Mg₂Sn phase-containing structure 4 isnot formed in the coating layer, rather, a Zn/Al/MgZn₂ ternary eutecticstructure 8 is formed together with a dendritic structure 5.

In addition, in a case where the cooling rate is not changed between thetemperature range from immediately after removal of the original sheetfor coating from the coating bath (that is, the coating bathtemperature) to 320° C., and the temperature range of from 320° C. to280° C., a sufficient amount of the laminar Mg₂Sn phase-containingstructure 4 is not necessarily formed.

For example, in the case of the cooling rate condition where the coolingrate is not changed in the above ranges, as shown in FIG. 4, a granularMg₂Sn phase-containing structure 4 is not formed in the coating layer 1.Instead, a structure 11 in which a plate-like Mg₂Sn phase (Mg₂Sn phasewith a thickness of more than 0.2 μm) is mixed in the Zn phase isformed. When the structure 11 is formed, the area fraction of theplate-like Mg₂Zn phase in the structure 11 is not less than 5% but lessthan 25%.

Although the detailed formation mechanism of this structure 11 is notclear, the following is conceivable. When the cooling rate A is 10° C./sor less, and the cooling rate B is less than 20° C./s, a Mg₂Sn phase iscoarsened from a laminar to plate-like form. When the cooling rate A isless than 10° C./s and the cooling rate B is 20° C./s or more, thesolidification behavior that naturally progresses in a nonequilibriumstate approaches an equilibrium state, and the Zn phase cannot grow in adendritic form. As a result, it is considered that a plate-like Mg₂Snphase having a thickness beyond 0.2 μm and an area fraction below 25% isformed.

The method of analyzing the structure of the hot-dip Zn coated steelsheet of the present disclosure will be described below.

The chemical component of a coating layer is measured by the followingmethod.

Firstly, calibration curves for quantitatively analyzing the respectiveelements are prepared by GDS (radio-frequency glow discharge-opticalemission spectroscopy). Thereafter, the chemical components in the depthdirection of the coating layer under test are measured.

Specifically, GDS (radio-frequency glow discharge-optical emissionspectroscopy) is performed on each standard sample such as a pure metalplate of each element to obtain in advance a calibration curve showingthe relationship between the elemental intensity plotted against eachelemental concentration.

Meanwhile, several 30 mm square pieces are taken from a sample of thecoated steel sheet under test, and used as test pieces for GDS. Argonion sputtering is performed from the surface layer of the coating layerto obtain an elemental intensity profile in the depth direction. Theobtained intensity profile is converted to the elemental concentrationwith the calibration curve.

In an analysis of the chemical composition by GDS, an analysis area of 4mmϕ or more is measured at 10 positions or more at a sputtering rate ina range of from 0.04 to 0.1 μm/sec. The average value of the elementalconcentration at each place is regarded as the elemental concentrationof the chemical composition.

However, at each GDS analysis position, in order to eliminate theinfluence of an oxide layer on the outermost layer, the 1 μm-deepsurface layer of the component profile is ignored, and the average valueof each elemental concentration in the depth range of from 1 to 10 μm (5μm width) is adopted.

The area fraction of a structure (provided, a Zn/Al/MgZn₂ ternaryeutectic structure is excluded) of the coating layer is measured by thefollowing method.

For measuring the area fraction of a structure of the coating layer, aFE-SEM equipped with an EDS (energy dispersive X-ray analyzer) is used.

A test piece having a cross section (cross section cut in the thicknessdirection of the coating layer) of 25 mm in the C direction and 15 mm inthe L direction is cut out from the coated steel sheet. The obtainedtest piece is embedded in a resin, and CP (cross session polisher)processing is applied to the cross section of the coating layer to bemeasured. After the CP processing, a backscattered electron image withan SEM and an element mapping image with an EDS of the cross section ofthe coating layer are created. For the backscattered electron image withan SEM and the element mapping image with an EDS, the magnification is5000×, and the visual field size is 50 μm long×200 μm wide.

Each region in a structure is identified based on the backscatteredelectron image with an SEM and the element mapping image with an EDS.

Next, in the backscattered electron image with an SEM, three values ofthe gray scale lightness, the hue, and the contrast value displayed byeach structure in the coating layer are determined. Since the threevalues of lightness, hue, and contrast value displayed by each structurereflect the atomic number of the element contained in each structure,that having higher contents of Al and Mg having a small atomic numbertends to display a black color, and that richer in Zn tends to display awhite color.

A computer image processing is performed such that a range correspondingto the 3 values to be displayed by each structure included in thecoating layer exhibits a specific color (for example, only a specificstructure is exhibited as a white image, and then the area (number ofpixels), etc. of each structure in the visual field is calculated). Byperforming this image processing for each phase, the area fraction ofeach structure in the coating layer occupied in the backscatteredelectron image from an SEM is determined.

The area fraction of each structure of the coating layer is defined asthe average value of the area fractions of the structure measured foreach of five visual fields in an optional cross section (cross sectioncut in the thickness direction of the coating layer) according to theabove-described operation.

In this regard, the area fraction of a laminar Mg₂Sn phase-containingstructure is the area fraction of a Zn phase, in which a laminar Mg₂Snphase having a thickness of less than 1 μm exists dividing the Zn phaseinto a plurality of regions, in a Zn phase region, provided that thearea of the laminar Mg₂Sn phase is also counted.

The area fraction of a dendritic structure is the area fraction of theregion occupied by of a solid solution of Zn and Al (the structureshowing the Al concentration of from 15 to 85%, and the Zn concentrationof from 15 to 85%).

The area fraction of a massive MgZn₂ phase is the area fraction of theMgZn₂ phase having an equivalent circle diameter of 1 μm or more.

The area fraction of a massive Zn phase is the area fraction of the Znphase having an equivalent circle diameter of 2 μm or more.

The area fraction occupied by the laminar Mg₂Sn phase with respect tothe laminar Mg₂Sn phase-containing structure (that is, the Zn phase andthe laminar Mg₂Sn phase) is measured in the same manner as above exceptthat the measuring object is changed to an SEM backscattered electronimage of a cross section of the coating layer for a visual field in asize of 12 μm×12 μm at a magnification of 10000×.

The average thickness of the laminar Mg₂Sn phase is calculated as anaverage value of the thicknesses of the laminar Mg₂Sn phase measured infive visual fields (5 positions in each visual field) of the same SEMbackscattered electron image.

A Zn/Al/MgZn₂ ternary eutectic structure in the coating layer isidentified and its area fraction is measured by the following method.

First, a structure in an SEM backscattered electron image, in whichthree phases of an Al phase, a Zn phase, and a MgZn₂ phase have formed aeutectic, is identified by the same method as the measurement of thearea fraction of each structure in the coating layer. A part of thestructure is observed by means of a rectangular visual field in a sizeof 3 μm×4 μm (diagonal: 5 μm) at a magnification of 30000× (see FIG. 5).In doing so, when two diagonals are drawn in the rectangular visualfield, in a case each diagonal crosses a Zn phase 5 or more times, and aMgZn₂ phase or an Al phase spreading around the Zn phase 5 or moretimes, the structure is judged as a ternary eutectic structure. Thisjudgment is based on the fact that a ternary eutectic structure ischaracterized by a “structure in which each of 3 phases is finelydispersed”.

In this regard, in a case where a ternary eutectic structure cannotextend to cover the region of 3 μm×4 μm due to possible unevendistribution of the ternary eutectic structure, or difficulty in forminga ternary eutectic structure, the structure may be divided into a 1μm-square grid-like pattern, and when each phase is included within asingle grid in a number of 1 or more respectively, it may be judged as aternary eutectic structure.

Next, on the same SEM backscattered electron image as the measurement ofthe area fraction of each structure in the coating layer (magnificationof 5000, visual field size: 50 μm long×200 μm wide), the above operationis repeated to grasp the outline (region) of the ternary eutecticstructure while confirming the continuity of the ternary eutecticstructure. Then, the area fraction of the grasped ternary eutecticstructure in the coating layer occupied in the SEM backscatteredelectron image is determined.

The area fraction of the ternary eutectic structure is defined as theaverage value of the area fraction of the ternary eutectic structureobtained in at least five visual fields in an optional cross section(cross section cut in the thickness direction of the coating layer)according to the above-described operation.

The average equivalent circle diameters of a massive MgZn₂ phase and amassive Zn phase are measured by the following method.

In the SEM backscattered electron image, in which each structure hasbeen identified in measuring the area fraction of the above structure,the top five equivalent circle diameters are selected with respect toeach identified phase type. Then, this operation is performed for fivevisual fields, and the arithmetic average of totally 25 equivalentcircle diameters is defined as the average equivalent circle diameter ofa massive MgZn₂ phase, or a massive Zn phase.

The thickness of an interfacial alloy layer composed of an Al—Feintermetallic compound is measured as follows.

In the SEM backscattered electron image, in which each structure hasbeen identified in measuring the area fraction of the above structure(magnification of 5000×, visual field size: 50 μm long×200 μm wide,provided that the visual field include a recognizable interfacial alloylayer), the thickness of the identified interfacial alloy layer ismeasured at each of optional 5 positions. The arithmetic average of thedata at 5 positions is defined as the thickness of the interfacial alloylayer.

A post-treatment applicable to the coated steel sheet of the presentdisclosure will be described below.

In the coated steel sheet of the present disclosure, a film may beformed on the coating layer. The film may be constituted with one ormore layers. Examples of the type of a film directly on the coatinglayer include a chromate film, a phosphate film, and a chromate-freefilm. As a chromate treatment, a phosphate treatment, or a chromate-freetreatment for forming the above film may be performed by known methods.

As for the chromate treatment, there are an electrolytic chromatetreatment in which a chromate film is formed by electrolysis, a reactiontype chromate treatment in which a film is formed utilizing a reactionwith a material, and a surplus treatment solution is washed away, and acoating type chromate treatment in which a treatment solution is coatedon an object and then dried without washing with water to form a film.Any of the above may be used.

Examples of the electrolytic chromate treatment include electrolyticchromate treatments using chromic acid, silica sol, a resin (such asphosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylicemulsion, carboxylated styrene butadiene latex, ordiisopropanolamine-modified epoxy resin), and hard silica.

Examples of the phosphate treatment include a zinc phosphate treatment,a calcium zinc phosphate treatment, and a manganese phosphate treatment.

The chromate-free treatment is particularly preferable because it isenvironmentally-friendly. In the chromate-free treatment, there are anelectrolytic chromate-free treatment in which a chromate film is formedby electrolysis, a reaction type chromate-free treatment in which a filmis formed utilizing a reaction with a material, and a surplus treatmentsolution is washed away, and a coating type chromate-free treatment inwhich a treatment solution is coated on an object and then dried withoutwashing with water to form a film. Any of the treatments may be adopted.

Furthermore, one, or two or more layers of organic resin films may beprovided on the film directly on the coating layer. The organic resin isnot limited to a specific type, and examples thereof include a polyesterresin, a polyurethane resin, an epoxy resin, an acrylic resin, apolyolefin resin, and modified products of these resins. In this regard,the modified product refers to a resin to be obtained by reacting afunctional group included in the structure of any of the above resinswith another compound (monomer, crosslinking agent, or the like) havinga functional group reactive with said functional group in the structure.

As such an organic resin, a single, or a mixture of two or more organicresins (not modified) may be used, or a single, or a mixture of two ormore organic resins to be obtained by modifying at least one otherorganic resin in the presence of at least one organic resin may be used.In addition, the organic resin film may contain an optional coloringpigment or rust prevention pigment. It may be also used after it isformed into an aqueous system by dissolving or dispersing it in water.

EXAMPLES

Example that is an embodiment of the present disclosure will bedescribed below.

As the coating bath, a coating bath in which the components were soadjusted that the chemical composition of the coating layer had thechemical composition shown in Table 1 was prepared. The coating bathtemperature was selected in the range of from 465 to 595° C.corresponding to the composition. As an original sheet for coating, ahot rolled steel sheet (carbon concentration 0.2%) having a sheetthickness of 0.8 mm was used. The original sheet was cut into a size of100 mm×200 mm, and then coated with a self-made batchwise hot-dip metalcoating test machine. The sheet temperature was monitored using athermocouple spot-welded at the center of the original sheet forcoating. In a case where Formula (1), which is relevant to thecomposition balance between Mg and Sn disclosed herein, was satisfied,OK was entered in Table 1, and when the same was not satisfied, NG wasentered.

Before immersion in the coating bath, the surface of the original sheetfor coating was reduced by a N₂-5% H₂ gas at 800° C. in a furnace withan oxygen concentration of 20 ppm or less, cooled with a N₂ gas, andwhen the temperature of the sheet to be immersed reached the bathtemperature +20° C., the sheet was immersed in the coating bath forabout 3 sec. After immersion in the coating bath, the sheet was pulledup at a pulling rate of 100 mm/sec. At the time of pulling up, thecoating amount was adjusted with an N₂ wiping gas.

After the steel sheet was taking out of the coating bath, the coatinglayer was cooled from the temperature of the coating bath to roomtemperature under the conditions written in Table 1 to produce a coatedsteel sheet.

Further, a commercial hot-dip zinc-coated steel sheet (No. 101 in Table1), alloyed zinc-coated steel sheet (No. 102 in Table 1), andelectrozinc-coated steel sheet (No. 103 in Table 1) were also preparedand subjected to the above evaluation.

TABLE 1 Cooling condition Average Chemical composition of coating layer(% by mass) cooling Average Total rate from cooling amount MeltingCoating coating rate from of point of bath bath temp. 320° C. tooptional Optional Formula coating temp. to 320° C. 280° C. Class No ZnAl Mg Sn Si elements element (1) (° C.) (° C.) (° C.)/sec) (° C.)/sec) C1 84.3 14 0.5 1.1 0.1 0 OK 440 460 10 40 C 2 83.8 15 0.4 0.7 0.1 0 OK445 465 10 40 E 3 83.7 15 0.5 0.5 0.1 0.2 OK 445 465 10 40 E 4 83.4 150.5 1 0.1 0 OK 445 465 10 40 E 5 83.5 15 0.5 1 0.05 0 OK 445 465 10 40 C6 83.1 15 0.5 1.2 0.2 0 OK 445 465 10 10 E 7 78.4 20 0.5 1 0.1 0 OK 475495 10 40 C 8 76.3 22 0.5 1.2 0.01 0 OK 480 500 10 40 E 9 75.3 22 0.5 20.2 0 NG 480 500 10 40 E 10 76.1 22 0.5 1.2 0.2 0 OK 480 500 10 40 E 1176.3 22 0.5 1 0.2 0 OK 480 500 10 40 E 12 76.3 22 0.5 1 0.2 0 OK 480 50010 40 C 13 76.3 22 0.5 1 0.2 0 OK 480 500 40 40 E 14 73.1 25 0.5 1.2 0.20 OK 487 507 10 40 E 15 69.0 25 0.5 1.2 0.2 4.1 Bi: 4.1 OK 488 508 10 40E 16 68.3 30 0.5 1 0.2 0 OK 481 501 10 150 C 17 69.2 30 0.5 0.1 0.2 0 OK510 530 10 40 E 18 58.3 40 0.5 1 0.2 0 OK 540 560 10 40 E 19 46.0 51 0.51 1.5 0 OK 565 585 10 40 E 20 38.1 60 0.5 1.2 0.2 0 OK 573 593 10 40 C21 36.1 62 0.5 1.2 0.2 0 OK 575 595 10 40 C 22 83.5 14 1 1 0.5 0 OK 430450 10 40 E 23 82.8 15 1 1 0.2 0 OK 445 465 10 40 C 24 81.8 17 1 0 0.2 0NG 456 476 10 40 E 25 77.8 20 1 1 0.2 0 OK 475 495 10 40 E 26 75.8 22 11 0.2 0 OK 480 500 10 40 E 27 74.3 22 1 2.5 0.2 0 OK 480 500 10 40 C 2876.8 22 1 0 0.2 0 NG 480 500 10 40 C 29 74.4 22 1 2.4 0.2 0 OK 480 50010 19 E 30 72.1 24 1 2.4 0.5 0 OK 485 505 10 40 C 31 72.1 24 1 2.4 0.5 0OK 485 505 20 20 E 32 70.1 26 1 2.4 0.5 0 OK 490 510 10 40 E 33 70.1 261 2.7 0.2 0 NG 490 510 10 40 E 34 68.4 28 1 2.4 0.2 0 OK 505 525 10 40 C35 67.6 29 1 2.4 0 0 OK 505 525 10 40 E 36 66.4 30 1 2.4 0.2 0 OK 510530 10 40 E 37 65.8 30 1 2.6 0.2 0.4 Pb: 0.1, OK 510 530 10 40 In: 0.3 E38 58.4 38 1 2.4 0.2 0 OK 535 555 10 40 E 39 56.4 40 1 2.4 0.2 0 OK 540560 10 20 E 40 56.2 40 1 2.4 0.1 0.3 B: 0.1, OK 540 560 10 20 V: 0.2 E41 51.4 45 1 2.4 0.2 0 OK 555 575 10 20 E 42 45.4 51 1 2.4 0.2 0 OK 565585 10 20 E 43 36.4 60 1 2.4 0.2 0 OK 573 593 10 20 C 44 34.4 62 1 2.40.2 0 OK 575 595 10 40 C 45 34.6 60 1 2.4 2 0 OK 573 593 10 40 C 46 78.814 2 5 0.2 0 OK 440 460 10 40 E 47 78.0 15 2 5 0.05 0 OK 445 465 10 40 E48 70.8 22 2 5 0.2 0 OK 480 500 10 40 C 49 71.0 22 2 5 0.01 0 OK 480 50010 40 E 50 68.8 22 2 7 0.2 0 NG 480 500 10 40 E 51 62.8 30 2 5 0.2 0 OK510 530 10 40 E 52 57.8 35 2 5 0.2 0 OK 528 548 10 40 C 53 52.6 40 2 41.4 0 OK 540 560 30 30 E 54 51.6 40 2 5 1.4 0 OK 540 560 10 40 E 55 47.845 2 5 0.2 0 OK 555 575 10 40 E 56 41.8 51 2 5 0.2 0 OK 565 585 10 40 E57 32.8 60 2 5 0.2 0 OK 573 593 10 40 C 58 30.8 62 2 5 0.2 0 OK 575 59510 40 E 59 74.8 15 3 7 0.2 0 OK 445 465 10 40 E 60 69.8 20 3 7 0.2 0 OK475 495 10 40 E 61 67.8 22 3 7 0.2 0 OK 480 500 10 40 E 62 64.8 22 4 90.2 0 OK 480 500 10 40 E 63 65.8 22 3 9 0.2 0 NG 480 500 10 40 E 64 64.825 3 7 0.2 0 OK 488 508 10 40 C 65 64.8 25 3 7 0.2 0 OK 488 508 40 40 E66 59.8 30 3 7 0.2 0 OK 510 530 10 40 E 67 54.8 35 3 7 0.2 0 OK 528 54810 40 E 68 49.8 40 3 7 0.2 0 OK 555 575 10 40 E 69 44.8 45 3 7 0.2 0 OK555 575 10 40 E 70 29.8 60 3 7 0.2 0 OK 573 593 10 40 C 71 27.6 62 3 7.20.2 0 OK 575 595 10 40 C 72 74.8 14 5 6 0.2 0 OK 440 460 10 20 E 73 71.815 5 8 0.2 0 OK 445 465 10 20 E 74 73.1 20 5 1.5 0.2 0.2 La: 0.1, OK 475495 10 20 Ca: 0.1 E 75 69.1 25 4 1.5 0.2 0.2 Ce: 0.2 OK 488 508 10 40 C76 62.8 25 5 7 0.2 0 OK 488 508 40 40 E 77 57.8 30 5 7 0.2 0 OK 510 53010 40 E 78 48.8 40 5 6 0.2 0 OK 555 575 10 40 E 79 38.5 50 4 7 0.2 0.3Cr: 0.1, OK 565 585 10 40 Cu: 0.2 E 80 27.8 60 5 7 0.2 0 OK 573 593 1040 C 81 27.8 62 5 5 0.2 0 OK 575 595 10 40 E 82 77.3 15 6 1.5 0.2 0 OK445 465 10 40 E 83 71.3 20 7 1.5 0.2 0 OK 475 495 10 40 E 84 67.3 25 61.5 0.2 0 NG 488 508 10 40 C 85 64.8 25 6 4 0.2 0 OK 488 508 40 40 C 8663.5 25 6 5 0.2 0.3 Y: 0.2, NG 488 508 10 40 Sb: 0.1 E 87 58.8 30 6 50.2 0 NG 510 530 10 40 E 88 48.8 40 7 4 0.2 0 NG 555 575 10 40 E 89 36.650 7 6 0.2 0.2 Sr: 0.2 NG 565 585 10 40 E 90 26.8 60 6 7 0.2 0 OK 573593 10 40 C 91 74.8 14 6 5 0.2 0 NG 575 595 10 40 E 92 69.6 15 7 8 0.20.2 Mn: 0.1, OK 445 465 10 40 Ni: 0.1 E 93 63.8 20 7 9 0.2 0 OK 475 49510 40 E 94 54.8 25 8 12 0.2 0 OK 488 508 10 40 C 95 59.8 25 8 7 0.2 0 OK488 508 20 20 E 96 52.6 30 8 9 0.2 0.2 Co; 0.1, OK 510 530 10 40 Ti :0.1E 97 36.8 40 8 15 0.2 0 OK 555 575 10 40 E 98 23.8 50 8 18 0.2 0 OK 565585 10 40 E 99 11.8 60 8 20 0.2 0 OK 573 593 10 40 C 100 12.8 60 9 180.2 0 OK 573 593 10 40 C 101 Commercia Zn-coated steel sheet 102 AlloyedZn-coated steel sheet 103 ElectroZn-coated steel sheet Structuralconstitution of coating layer Zn/Al/MgZn₂ Laminar phase Dendriticternary Massive MgZn₂ phase Massive Zn phase containing structureeutectic Equivalent Equivalent structure Area structure circle Areacircle Area Area fraction fraction Area fraction diameter fractiondiameter fraction Class No (%) (%) (%) (μm) (%) (μm) (%) C 1 54 28 0 — 04 18 C 2 40 36 0 — 0 5 24 E 3 47 36 0 — 0 8 17 E 4 47 38 0 — 0 8 15 E 545 35 0 — 0 4 20 C 6 4 37 0 — 0 2 19 E 7 39 45 0 — 0 5 16 C 8 51 38 0 —0 4 11 E 9 28 56 0 — 0 4 10 E 10 28 61 0 — 0 11  11 E 11 29 62 0 — 0 5 9E 12 26 64 0 — 0 4 10 C 13 0 60 0 — 0 8 13 E 14 23 69 0 — 0 3 8 E 15 2468 0 — 0 3 8 E 16 24 71 0 — 0 7 5 C 17 2 68 25 — 0 2 5 E 18 23 72 0 — 05 5 E 19 17 80 0 — 0 7 3 E 20 5 95 0 — 0 — 0 C 21 4 96 0 — 0 — 0 C 22 5934 0 — 0 5 7 E 23 65 35 0 — 0 — 0 C 24 0 42 39 — 13 2 6 E 25 55 45 0 — 0— 0 E 26 39 60 1 — 0 — 0 E 27 35 65 0 — 0 — 0 C 28 0 54 44 — 0 3 2 C 290 63 0 — 0 — 0 E 30 32 68 0 — 0 — 0 C 31 0 60 0 — 0 8 13 E 32 30 70 0 —0 — 0 E 33 25 72 0 — 0 — 0 E 34 26 74 0 — 0 — 0 C 35 33 67 0 — 0 — 0 E36 30 70 0 — 0 — 0 E 37 29 71 0 — 0 — 0 E 38 27 72 0 — 0 2 1 E 39 23 770 — 0 — 0 E 40 23 77 0 — 0 — 0 E 41 22 77 0 — 0 1 1 E 42 11 89 0 — 0 — 0E 43 5 95 0 — 0 — 0 C 44 2 98 0 — 0 — 0 C 45 5 93 0 — 0 — 0 C 46 60 33 0— 0 2 7 E 47 61 36 0 1 2 — 1 E 48 35 65 0 — 0 — 0 C 49 50 40 0 — 0 3 10E 50 25 58 0 — 0 4 9 E 51 29 71 0 — 0 — 0 E 52 25 74 0 1 1 — 0 C 53 0 600 — 0 8 13 E 54 24 73 0 1 2 — 0 E 55 20 77 0 2 3 — 0 E 56 9 87 0 2 4 — 0E 57 5 92 0 1 3 — 0 C 58 5 95 0 — 0 — 0 E 59 60 35 0 1 5 — 0 E 60 50 490 2 1 — 0 E 61 31 65 0 2 4 — 0 E 62 29 60 0 2 9 — 0 E 63 25 70 0 — 0 — 0E 64 37 61 0 2 2 — 0 C 65 0 60 0 — 0 8 13 E 66 32 68 0 — 0 — 0 E 67 2971 0 — 0 — 0 E 68 27 72 0 1 1 — 0 E 69 23 73 0 2 4 — 0 E 70 5 92 0 2 3 —0 C 71 3 97 0 — 0 — 0 C 72 55 37 0 2 8 — 0 E 73 57 35 0 4 8 — 0 E 74 4251 0 3 7 — 0 E 75 38 55 0 3 7 — 0 C 76 0 75 0 2 0 — 0 E 77 31 61 0 2 8 —0 E 78 24 69 0 5 7 — 0 E 79 6 84 0 2 8 — 0 E 80 5 86 0 2 9 — 0 C 81 3 880 3 9 — 0 E 82 60 29 0 1 11 — 0 E 83 50 39 0 2 11 — 0 E 84 36 52 0 2 12— 0 C 85 0 65 0 5 11 — 0 C 86 29 62 0 5 9 — 0 E 87 27 61 0 6 12 — 0 E 8822 67 0 8 11 — 0 E 89 6 78 0 8 14 — 0 E 90 5 82 1 8 12 — 0 C 91 51 38 05 11 — 0 E 92 53 30 0 9 17 — 0 E 93 47 39 0 5 14 — 0 E 94 32 51 0 9 17 —0 C 95 0 59 0 11  15 — 0 E 96 25 56 0 8 19 — 0 E 97 22 59 0 4 19 — 0 E98 6 72 0 12  20 — 0 E 99 5 75 0 15  20 — 0 C 100 5 70 0 15  25 — 0 C101 Commercial Zn-coated steel sheet 102 Alloyed Zn-coated steel sheet103 ElectroZn-coated steel sheet Structural constitution of coatinglayer Zn phase + plate- like Sn phase Si phase Mg₂Si phase Mg₂Sn Equiv-Equiv- Equiv- Other Interfacial phase alent alent alent phase alloy Areacircle Area circle Area circle Area Area layer fraction diameterfraction diameter fraction diameter fraction fraction Thickness ClassNo. (%) (μm) (%) (μm) (%) (μm) (%) (%) (μm) C 1 0 — 0 — 0 — 0 0 0.3 C 20 — 0 — 0 — 0 0 0.3 E 3 0 — 0 — 0 — 0 0 0.3 E 4 0 — 0 — 0 — 0 0 0.3 E 50 — 0 — 0 — 0 0 0.3 C 6 40 — 0 — 0 — 0 0 0.5 E 7 0 — 0 — 0 — 0 0 0.6 C 80 — 0 — 0 — 0 0 1.7 E 9 0 1 6 — 0 — 0 0 0.2 E 10 0 — 0 — 0 — 0 0 0.5 E11 0 — 0 — 0 — 0 0 0.4 E 12 0 — 0 — 0 — 0 0 0.3 C 13 27 — 0 — 0 — 0 00.3 E 14 0 — 0 — 0 — 0 0 0.2 E 15 0 — 0 — 0 — 0 0 0.2 E 16 0 — 0 — 0 — 00 0.8 C 17 0 — 0 — 0 — 0 0 0.4 E 18 0 — 0 — 0 — 0 0 0.7 E 19 0 — 0 — 0 —0 0 1.2 E 20 0 — 0 — 0 — 0 0 1.1 C 21 0 — 0 — 0 — 0 0 1.8 C 22 0 — 0 — 0— 0 0 0.2 E 23 0 — 0 — 0 — 0 0 0.1 C 24 0 — 0 — 0 — 0 0 0.4 E 25 0 — 0 —0 — 0 0 0.1 E 26 0 — 0 — 0 — 0 0 0.2 E 27 0 — 0 — 0 — 0 0 0.1 C 28 0 — 0— 0 — 0 0 0.2 C 29 37 — 0 — 0 — 0 0 0.4 E 30 0 — 0 — 0 — 0 0 0.1 C 31 27— 0 — 0 — 0 0 0.1 E 32 0 — 0 — 0 — 0 0 0.3 E 33 0 1 3 — 0 — 0 0 0.5 E 340 — 0 — 0 — 0 0 0.6 C 35 0 — 0 — 0 — 0 0 1.7 E 36 0 — 0 — 0 — 0 0 0.9 E37 0 — 0 — 0 — 0 0 0.9 E 38 0 — 0 — 0 — 0 0 1.1 E 39 0 — 0 — 0 — 0 0 1 E40 0 — 0 — 0 — 0 0 1 E 41 0 — 0 — 0 — 0 0 1.1 E 42 0 — 0 — 0 — 0 0 1.2 E43 0 — 0 — 0 — 0 0 1.4 C 44 0 — 0 — 0 — 0 0 2 C 45 0 — 0 1 2 — 0 0 0.1 C46 0 — 0 — 0 — 0 0 0.3 E 47 0 — 0 — 0 — 0 0 0.2 E 48 0 — 0 — 0 — 0 0 0.1C 49 0 — 0 — 0 — 0 0 1.9 E 50 0 1 8 — 0 — 0 0 0.2 E 51 0 — 0 — 0 — 0 00.1 E 52 0 — 0 — 0 — 0 0 0.3 C 53 27 — 0 — 0 — 0 0 0.3 E 54 0 — 0 — 0 11 0 0.5 E 55 0 — 0 — 0 — 0 0 0.9 E 56 0 — 0 — 0 — 0 0 0.8 E 57 0 — 0 — 0— 0 0 1.3 C 58 0 — 0 — 0 — 0 0 2.1 E 59 0 — 0 — 0 — 0 0 0.1 E 60 0 — 0 —0 — 0 0 0.2 E 61 0 — 0 — 0 — 0 0 0.5 E 62 0 2 2 — 0 — 0 0 0.5 E 63 0 1 5— 0 — 0 0 0.5 E 64 0 — 0 — 0 — 0 0 0.2 C 65 27 — 0 — 0 — 0 0 0.2 E 66 0— 0 — 0 — 0 0 0.3 E 67 0 — 0 — 0 — 0 0 0.8 E 68 0 — 0 — 0 — 0 0 0.9 E 690 — 0 — 0 — 0 0 1.5 E 70 0 — 0 — 0 — 0 0 1.3 C 71 0 — 0 — 0 — 0 0 2.2 C72 0 — 0 — 0 — 0 0 0.3 E 73 0 — 0 — 0 — 0 0 0.1 E 74 0 — 0 — 0 — 0 0 0.2E 75 0 — 0 — 0 — 0 0 0.2 C 76 25 — 0 — 0 — 0 0 0.7 E 77 0 — 0 — 0 — 0 00.3 E 78 0 — 0 — 0 — 0 0 1.3 E 79 0 — 0 — 0 1 2 0 1.4 E 80 0 — 0 — 0 — 00 1.3 C 81 0 — 0 — 0 — 0 0 1.7 E 82 0 — 0 — 0 — 0 0 0.1 E 83 0 — 0 — 0 —0 0 0.2 E 84 0 — 0 — 0 — 0 0 0.2 C 85 24 — 0 — 0 — 0 0 0.7 C 86 0 — 0 —0 — 0 0 0.2 E 87 0 — 0 — 0 — 0 0 0.3 E 88 0 — 0 — 0 — 0 0 1.3 E 89 0 — 0— 0 1 2 0 1.4 E 90 0 — 0 — 0 — 0 0 1.3 C 91 0 — 0 — 0 — 0 0 0.3 E 92 0 —0 — 0 — 0 0 0.1 E 93 0 — 0 — 0 — 0 0 0.2 E 94 0 — 0 — 0 — 0 0 0.2 C 9526 — 0 — 0 — 0 0 0.7 E 96 0 — 0 — 0 — 0 0 0.3 E 97 0 — 0 — 0 — 0 0 1.3 E98 0 — 0 — 0 1 2 0 1.4 E 99 0 — 0 — 0 — 0 0 1.3 C 100 0 — 0 — 0 — 0 01.5 C 101 Commercial Zn-coated steel sheet 102 Alloyed Zn-coated steelsheet 103 ElectroZn-coated steel sheet Corrosion resistance Sacrificialcorrosion Resist- Resist- Bending after painting protection propertyance ance workability 30 60 90 150 60 120 240 360 to to Class No. 2T 4T6T cycles cycles cycles cycles cycles cycles cycles cycles chippingseizure C 1 C C C B B C D B B C D B D C 2 B B B B B B C B B B C A D E 3B B B A A A B A A A B A B E 4 B B B A A A B A A A B A B E 5 B B B A A AB A A A B A B C 6 C C C A A B C A A B C B B E 7 A A A A A A B A A A B AB C 8 D D D C C D D C C D D D A E 9 A A A A B C D A B C D A A E 10 A A AA A A B A A A B A B E 11 A A A A A A B A A A B A B E 12 A A A A A A B AA A B A A C 13 C C C A A B C A A B C B A E 14 A A A A A A B A A A B A AE 15 A A A A A A B A A A B A A E 16 A A A A A A B A A A B A A C 17 D D DC C D D C C D D B A E 18 A A A A A A B A A A B A A E 19 A A A A A A B AA B B A A E 20 A A A A A B B A A B B A A C 21 C C C A A B C A A B C C AC 22 C C C B B C D B B C D B D E 23 B B B A A A A A A A A A B C 24 D D DC C D D C C D D B A E 25 A A A A A A A A A A A A A E 26 A A A A A A A AA A A A A E 27 A A A A A A A A A A A A A C 28 D D D C C D D C C D D B AC 29 C C C A A B C A A B C B B E 30 A A A A A A A A A A A A A C 31 C C CA A B C A A B C B A E 32 A A A A A A A A A A A A A E 33 A A A A B C D AB C D A A E 34 A A A A A A B A A A B A A C 35 C C C A A B C A A B C C AE 36 A A A A A A B A A A B A A E 37 A A A A A A B A A A B A A E 38 A A AA A A B A A A B A A E 39 A A A A A A B A A A B A A E 40 A A A A A A B AA A B A A E 41 A A A A A A B A A A B A A E 42 A A A A A B B A A B B A AE 43 A A A A A B B A A B B A A C 44 C C C C A B C A A B C C A C 45 C C CC B C D B B C D A A C 46 C C C C B C D B B C D B D E 47 B B B A A A A AA A A A B E 48 A A A A A A A A A A A A A C 49 D D D C C D D C C D D D AE 50 A A A A B C D A B C D A A E 51 A A A A A A B A A A B A A E 52 A A AA A A B A A A B A A C 53 C C C A A B C A A B C B A E 54 A A A A A A B AA A B A A E 55 A A A A A A B A A A B A A E 56 B B B A A B B A A B B A AE 57 A A A A A B B A A B B A A C 58 C C C A A B C A A B C C A E 59 B B BA A A A A A A A A B E 60 A A A A A A A A A A A A A E 61 A A A A A A A AA A A A A E 62 D B A A B C D A B C D B A E 63 A A A A B C D A B C D A AE 64 A A A A A A A A A A A A A C 65 D C C B B C D B B C D B A E 66 A A AA A A A A A A A A A E 67 A A A A A A A A A A A A A E 68 A A A A A A B AA A B A A E 69 A A A A A A B A A A B A A E 70 A A A A A B B A A B B A AC 71 C C C A A B C A A B C C A C 72 D C B B B C D B B B B B D E 73 D B AA A A A A A A A A B E 74 D B A A A A A A A A A A A E 75 D B A A A A A AA A A A A C 76 D D C A A B C B B B B B A E 77 D B A A A A A A A A A A AE 78 D B A A A A B A A A A A A E 79 D B A A A B B A A A A A A E 80 D B AA A B B A A A A A A C 81 D C C A A B C C C C C C A E 82 D B A A A A A AA A A A B E 83 D B A A A A A A A A A A A E 84 D B A A A A A A A A A A AC 85 D D C A A B C B B B B B A C 86 A A A A A A B A A A B A A E 87 D B AA A A B A A A A A A E 88 D B A A A A B A A A A A A E 89 D B A A A B B AA A A A A E 90 D B A A A B B A A A A A A C 91 D C B B B C D B B B B B DE 92 D B A A A A A A A A A A B E 93 D B A A A A A A A A A A A E 94 D B AA A A A A A A A A A C 95 D D C A A B C B B B B B A E 96 D B A A A A B AA A A A A E 97 D B A A A A B A A A A A A E 98 D B A A A B B A A A A A AE 99 D B A A A B B A A A A A A C 100 D D C A A B C C C C C C A C 101 C CC C C C D B B B B B D 102 D D D C D D D D D D D D A 103 B B B C C C D CC C C C D

The following measurements and evaluations were carried out for thecoated steel sheets produced in the respective examples, which aresummarized in Table 1 above.

—Measurement of Area Fraction of Each Structure—

The area fractions of the following structures of the coating layer ofthe obtained coated steel sheet were measured according to the methoddescribed above.

-   -   Laminar Mg₂Sn phase-containing structure (denoted as “laminar        phase-containing structure” in the Table)    -   Dendritic structure    -   Zn/Al/MgZn₂ ternary eutectic structure    -   Massive MgZn₂ phase with an equivalent circle diameter of 1 μm        or more    -   Massive Zn phase with an equivalent circle diameter of 2 μm or        more    -   Plate-like Mg2Sn phase-containing structure in which a        plate-like Mg₂Sn phase is mixed in a Zn phase (denoted as “Zn        phase+plate-like Mg₂Sn phase” in the Table)    -   Sn phase    -   Si phase    -   Mg₂Si phase    -   Intermetallic compound phase other than the above structures        (denoted as “other phase” in the Table)

—Measurement of Average Equivalent Circle Diameter of Each Structure—

The average equivalent circle diameters of the following structures ofthe coating layer of the obtained coated steel sheet were measuredaccording to the method described above. In this regard, in Table 1, theaverage equivalent circle diameter is denoted as “equivalent circlediameter”.

-   -   Massive MgZn₂ phase with an equivalent circle diameter of 1 μm        or more    -   Massive Zn phase with an equivalent circle diameter of 2 μm or        more    -   Sn phase    -   Si phase    -   Mg₂Si phase

—Measurement of Thickness of Interfacial Alloy Layer—

The thickness of the interfacial alloy layer of the obtained coatedsteel sheet was measured according to the method described above.

—Average Thickness and Area Fraction of Laminar MgSn Phase of LaminarMg₂Sn Phase-Containing Structure—

An SEM backscattered electron image (BSE image) of No. 26 shown in Table1 was obtained. The SEM backscattered electron image (BSE image) of No.26 shown in Table 1 is shown in FIG. 1 and FIG. 2. As apparent from FIG.1, the coating layer 1 was mainly constituted with granular Mg₂Snphase-containing structures 4 and dendritic structures 5. Then, theaverage thickness and the area fraction (the area fraction of thelaminar Mg₂Sn phase with respect to the laminar Mg₂Sn phase-containingstructure (that is, the Zn phase and the laminar Mg₂Sn phase) of alaminar Mg₂Sn phase 7 formed in the laminar Mg₂Sn phase-containingstructure 4 shown in FIG. 2 were examined.

Similarly, for other samples, the average thickness and the areafraction of the laminar Mg₂Sn phase were examined. As a result, typicalnumerical values of the average thickness and the area fraction of thelaminar Mg₂Sn phase formed in the laminar Mg₂Sn phase-containingstructure were as shown in the following Table 2.

TABLE 2 Thickness (nm) Area fraction (%) 40 10 50 25 80 30 75 45 100 36150 40 200 45

—Bending Workability—

Evaluation of the bending workability of the coating layer was performedas follows.

From the obtained coated steel sheet, a test piece of 30 mm in Cdirection×60 mm in L direction (L) was cut out. The test piece was bentby 180° in the C direction (1T bending), and the crest of the workedpart of the coating layer was observed with an SEM, and the number ofcracks present at the crest (1.6 mm×30 mm) was counted.

A test piece in which four test pieces with the same thickness weresandwiched inside, and a test piece in which six test pieces with thesame thickness were sandwiched inside were respectively bent in the Cdirection by 180° (6T bending and 6T bending). Similarly, the numbers ofcracks were counted.

In this regard, at least three samples of each coated steel sheet wereprepared, and the bending workability was evaluated by calculating theaverage number of the existing cracks. It can be so evaluated that thesmaller average number of the cracks indicates the better plasticdeformability and therefore the better bending workability.

The rating criteria were: in a case where the average number of crackwas 0, namely there was no crack, it was rated as “A”; in a case wherethe average number of cracks was from 1 to 20, it was rated as “B”; in acase where the average number of cracks was from 21 to 100, it was ratedas “C”; and in a case where the average number of cracks was 101 ormore, it was rated as “D”.

—Evaluation of Corrosion Resistance after Painting—

Evaluation of the corrosion resistance after painting of the coatinglayer was performed as follows.

From the obtained coated steel sheet, a test piece of 50 mm in Cdirection×100 mm in L direction was cut out. A Zn phosphoric acidtreatment (SD5350 system: Specifications of Nipponpaint IndustrialCoatings Co., Ltd.) was applied to the surface of the coating layer ofthe test piece.

Next, a 20 μm-thick paint film was formed on the Zn phosphoricacid-treated surface of the coating layer of the test piece byelectropainting (PN110 POWERNIX Gray: Specifications of NipponpaintIndustrial Coatings Co., Ltd.), and baked at a baking temperature of150° C. for 20 min to form an electrodeposited film.

Next, cross cuts (two cuts of 40×√2) reaching the steel substrate (steelsheet) were made in the electrodeposited film of the test piece.

The obtained test piece was subjected to a combined cyclic corrosiontest according to JASO (M609-91). And the maximum blistering widths at 8positions around the cross cuts respectively after execution of 30, 60,90, and 150 cycles were measured, and the average value was calculated.

The corrosion resistance after painting was rated by this blisteringwidth. The rating criteria were: respectively after execution of 30, 60,90, and 150 cycles according to JASO (M609-91), in a case where theblistering width from the cross cut was 1 mm or less, it was rated as“A”; in a case where the same was more than 1 mm but not more than 2 mm,it was rated as “B”, in a case where the same was more than 2 mm but notmore than 4 mm, it was rated as “C”; and in a case where red rustappeared, it was rated as “D”.

—Evaluation of Corrosion Resistance after Painting—

Evaluation of the sacrificial corrosion protection property of thecoating layer was performed as follows.

From the obtained coated steel sheet, a test piece of 50 mm in Cdirection×100 mm in L direction was cut out. A Zn phosphoric acidtreatment (SD5350 system: Specifications of Nipponpaint IndustrialCoatings Co., Ltd.) was applied to the surface of the coating layer ofthe test piece.

Next, electropainting (PN110 POWERNIX Gray: Specifications ofNipponpaint Industrial Coatings Co., Ltd.) was performed at 20 μm on theZn phosphoric acid-treated surface of the coating layer of the testpiece and the film was baked at a baking temperature of 150° C. for 20min to form a electrodeposited film.

Next, cross cuts (two cuts of 40×√2) reaching the steel substrate weremade in the electrodeposited film of the test piece.

The obtained test piece was subjected to a combined cyclic corrosiontest according to JASO (M609-91). And the erosion depth of the steelsubstrate was measured with a micrometer after each test of 30, 60, 90,or 150 cycles and the average value was calculated.

The corrosion resistance after painting was rated by this erosion depth.The rating criteria were: respectively after execution of 60, 120, 240,and 360 cycles according to JASO (M609-91), in a case where the erosiondepth from the cross cut was less than 0.1 mm, it was rated as “A”; in acase where the same was not less than 0.1 mm but less than 0.3 mm, itwas rated as “B”, in a case where the same was not less than 0.3 mm butless than 0.4 mm, it was rated as “C”; and in a case where the same wasnot less than 0.4 mm, it was rated as “D”.

—Evaluation of Resistance to Chipping—

Evaluation of the resistance to chipping of a coating layer wasperformed as follows.

A test piece with a coating layer, which surface was provided with anelectrodeposition coating, was prepared in the same manner as in theevaluation of corrosion resistance after painting. On theelectrodeposition coating surface, further intermediate painting, topcoat painting, and clear painting were conducted to form the respectivepaint films, such that the total film thickness became 40 μm.

Using a Gravel Test Instrument (manufactured by Suga Test InstrumentsCo., Ltd.) 100 g of No. 7 crushed stone was blasted against the paintfilm of the test piece cooled to −20° C. at a collision angle of 90°with an air pressure of 3.0 kg/cm² from a distance of 30 cm. Thendetached regions of the coating layer in the collided area were exposedusing an adhesive tape, and the diameters of the detached regions weremeasured. The top five largest detachment diameters were selected andthe average value thereof was regarded as the average detachmentdiameter.

The resistance to chipping was evaluated by this average detachmentdiameter. The smaller average detachment diameter means the betterresistance to chipping.

The rating criteria were: in a case where the average detachmentdiameter was less than 1.0 mm, it was rated as “A”; in a case where theaverage detachment diameter was not less than 1.0 mm but less than 1.5mm, it was rated as “B”; in a case where the average detachment diameterwas not less than 1.5 mm but less than 3.0, it was rated as “C”; and ina case where the average detachment diameter was not less than 3.0 mm,it was rated as “D”.

—Evaluation of Resistance to Seizure—

Evaluation of the resistance to seizure of a coating layer was performedas follows.

Each two test pieces of 80 mm in the C direction and 350 mm in the Ldirection were cut out from the obtained coated steel sheet. A draw-beadworking was applied to the two test pieces using jigs that simulate adie and a bead, such that sliding occurred over the length of 150 mm ormore between the coating forming layer forming surface of the test pieceand the die shoulder as well as the bead portion. In this case, theradii of curvature of the die shoulder and the bead portion, which wereused as jigs in the test, were 2 mmR and 5 mmR respectively, the pushingpressure of the die was 60 kN/m², and the drawing rate in the draw-beadworking was 2 m/min. Further, at the time of the test, a lubricating oil(550S, produced by Nihon Parkerizing Co., Ltd.) was applied on to boththe surfaces of the test piece at, in total, 10 mg/m².

Then, each two primary test pieces of 80 mm wide×350 mm long weresampled, and a draw-bead working was applied to them using jigs thatsimulate a die and a bead, such that sliding occurred over the length of150 mm or more between a surface treated surface of the steel sheet andthe die shoulder as well as the bead portion for evaluation of theresistance to seizure. In this case, the radii of curvature of the dieshoulder and the bead portion, which were used as jigs in the test, were2 mmR and 5 mmR respectively, the pushing pressure of the die was 60kN/m², and the drawing rate in the draw-bead working was 2 m/min.Further, at the time of the test, a lubricating oil (550S, produced byNihon Parkerizing Co., Ltd.) was applied on to both the surfaces of thetest piece at, in total, 0.5 g/m².

The rating criteria were: in a case where there was no visuallyrecognizable seizure between the coating layer and the die or bead, itwas rated as “A”; in a case where there was visually recognizable mildseizure between the coating layer and the die or bead, it was rated as“B”; and in a case where there was visually recognizable significantseizure between the coating layer and the die or bead, it was rated as“D”.

REFERENCE SIGNS LIST

-   -   1 Coating layer    -   2 Steel sheet    -   3 Interfacial alloy layer    -   4 Laminar Mg₂Sn phase-containing structure    -   5 Dendritic structure    -   6 Zn phase    -   7 Laminar Mg₂Sn phase    -   8 Zn/Al/MgZn₂ ternary eutectic structure    -   9 Massive Zn phase    -   10 Massive MgZn₂ phase    -   11 Structure in which plate-like Mg₂Sn phase is mixed in Zn        phase    -   20 Zn phase in Zn/Al/MgZn₂ ternary eutectic structure    -   21 MgZn₂ phase in Zn/Al/MgZn₂ ternary eutectic structure    -   22 Al phase in Zn/Al/MgZn₂ ternary eutectic structure

The disclosure of Japanese Patent Application 2017-053148 isincorporated herein by reference in its entirety.

All the literature, patent application, and technical standards citedherein are also herein incorporated to the same extent as provided forspecifically and severally with respect to an individual literature,patent application, and technical standard to the effect that the sameshould be so incorporated by reference.

The invention claimed is:
 1. A coated steel sheet comprising a steelsheet and a coating layer provided on at least a part of a surface ofthe steel sheet, wherein: the coating layer has a chemical compositioncomprising in terms of % by mass: Al: from 15% to 60%, Mg: from 0.5% to8.0%, Sn: from 0.5% to 20.0%, Si: from 0.05% to 1.50%, Bi: from 0% to5.0%, In: from 0% to 2.0%, Ca: from 0% to 3.0%, Y: from 0% to 0.5%, La:from 0% to 0.5%, Ce: from 0% to 0.5%, Cr: from 0% to 0.25%, Ti: from 0%to 0.25%, Ni: from 0% to 0.25%, Co: from 0% to 0.25%, V: from 0% to0.25%, Nb: from 0% to 0.25%, Cu: from 0% to 0.25%, Mn: from 0% to 0.25%,Sr: from 0% to 0.5%, Sb: from 0% to 0.5%, Pb: from 0% to 0.5%, B: from0% to 0.5%, and a balance comprising Zn and impurities, wherein: thecoating layer has a laminar Mg₂Sn phase-containing structure in an areafraction of from 5 to 65% measured in cross section thereof, and astructure containing a solid solution of Zn and Al, and the laminarMg₂Sn phase-containing structure is a structure constituted with a Znphase and a laminar Mg₂Sn phase having a thickness of less than 1 μm,and wherein the laminar Mg₂Sn phase exists dividing the Zn phase into aplurality of regions.
 2. The coated steel sheet according to claim 1,wherein a content of Mg is from 0.5% to 3.0%, and a content of Sn isfrom 1.0% to 7.5% in terms of % by mass.
 3. The coated steel sheetaccording to claim 2, wherein a content of Al is from 20% to 60%, acontent of Mg is from 1.0% to 2.0%, a content of Sn is from 1.0% to5.0%, and a content of Si is from 0.05% to 1.0% in terms of % by mass.4. The coated steel sheet according to claim 2, wherein a content of Snand a content of Mg satisfy the following Formula (1):Mg≤Sn≤2.5×Mg  Formula (1) wherein, in Formula (1), each atomic symbolindicates a content of the element in terms of % by mass.
 5. The coatedsteel sheet according to claim 2, wherein the area fraction of thelaminar Mg₂Sn phase-containing structure is from 20% to 60% measured incross section thereof.
 6. The coated steel sheet according to claim 1,wherein a content of Al is from 20% to 60%, a content of Mg is from 1.0%to 2.0%, a content of Sn is from 1.0% to 5.0%, and a content of Si isfrom 0.05% to 1.0% in terms of % by mass.
 7. The coated steel sheetaccording to claim 6, wherein a content of Sn and a content of Mgsatisfy the following Formula (1):Mg≤Sn≤2.5×Mg  Formula (1) wherein, in Formula (1), each atomic symbolindicates a content of the element in terms of % by mass.
 8. The coatedsteel sheet according to claim 6, wherein the area fraction of thelaminar Mg₂Sn phase-containing structure is from 20% to 60% measured incross section thereof.
 9. The coated steel sheet according to claim 1,wherein a content of Sn and a content of Mg satisfy the followingFormula (1):Mg≤Sn≤2.5×Mg  Formula (1) wherein, in Formula (1), each atomic symbolindicates a content of the element in terms of % by mass.
 10. The coatedsteel sheet according to claim 9, wherein the area fraction of thelaminar Mg₂Sn phase-containing structure is from 20% to 60% measured incross section thereof.
 11. The coated steel sheet according to claim 1,wherein the area fraction of the laminar Mg₂Sn phase-containingstructure is from 20% to 60% measured in cross section thereof.
 12. Thecoated steel sheet according to claim 1, wherein the area fraction ofthe laminar Mg₂Sn phase-containing structure is from 30% to 60% measuredin cross section thereof.
 13. The coated steel sheet according to claim1, wherein an area fraction of the structure containing a solid solutionof Zn and Al is from 35% to 95% measured in cross section thereof. 14.The coated steel sheet according to claim 1, wherein the coating layerhas a massive MgZn₂ phase with an equivalent circle diameter of 1 μm ormore in an area fraction of more than 0% and less than or equal to 20%measured in cross section thereof.
 15. The coated steel sheet accordingto claim 1, wherein the coating layer has a massive MgZn₂ phase with anequivalent circle diameter of 1 μm or more in an area fraction of morethan 0% and less than or equal to 5% measured in cross section thereof.16. The coated steel sheet according to claim 1, wherein the coatinglayer has a massive Zn phase with an equivalent circle diameter of 2 μmor more in an area fraction of more than 0% and less than or equal to20% measured in cross section thereof.
 17. The coated steel sheetaccording to claim 1, wherein the coating layer has a massive Zn phasewith an equivalent circle diameter of 2 μm or more in an area fractionof more than 0% and less than or equal to 10% measured in cross sectionthereof.
 18. The coated steel sheet according to claim 1, furthercomprising an interfacial alloy layer with a thickness of from 100 nm to1.5 μm consisting of an Al—Fe intermetallic compound between the steelsheet and the coating layer.